diff --git a/src/Digdir.Domain.Dialogporten.Application/ApplicationExtensions.cs b/src/Digdir.Domain.Dialogporten.Application/ApplicationExtensions.cs
index b962d8198..bdd505b52 100644
--- a/src/Digdir.Domain.Dialogporten.Application/ApplicationExtensions.cs
+++ b/src/Digdir.Domain.Dialogporten.Application/ApplicationExtensions.cs
@@ -2,6 +2,7 @@
 using Digdir.Domain.Dialogporten.Application.Common.Behaviours;
 using Digdir.Domain.Dialogporten.Application.Common.Extensions;
 using Digdir.Domain.Dialogporten.Application.Common.Extensions.OptionExtensions;
+using Digdir.Domain.Dialogporten.Application.Common.Services;
 using FluentValidation;
 using MediatR;
 using Microsoft.Extensions.Configuration;
@@ -43,6 +44,7 @@ public static IServiceCollection AddApplication(this IServiceCollection services
             .AddTransient<IUserOrganizationRegistry, UserOrganizationRegistry>()
             .AddTransient<IUserResourceRegistry, UserResourceRegistry>()
             .AddTransient<IUserNameRegistry, UserNameRegistry>()
+            .AddTransient<IDialogActivityService, DialogActivityService>()
             .AddTransient<IClock, Clock>()
             .AddTransient(typeof(IPipelineBehavior<,>), typeof(ValidationBehaviour<,>))
             .AddTransient(typeof(IPipelineBehavior<,>), typeof(DomainContextBehaviour<,>));
diff --git a/src/Digdir.Domain.Dialogporten.Application/Common/IUserOrganizationRegistry.cs b/src/Digdir.Domain.Dialogporten.Application/Common/IUserOrganizationRegistry.cs
index 99bb14be3..74904737d 100644
--- a/src/Digdir.Domain.Dialogporten.Application/Common/IUserOrganizationRegistry.cs
+++ b/src/Digdir.Domain.Dialogporten.Application/Common/IUserOrganizationRegistry.cs
@@ -7,6 +7,7 @@ namespace Digdir.Domain.Dialogporten.Application.Common;
 public interface IUserOrganizationRegistry
 {
     Task<string?> GetCurrentUserOrgShortName(CancellationToken cancellationToken);
+    Task<IList<OrganizationLongName>?> GetCurrentUserOrgLongNames(CancellationToken cancellationToken);
 }
 
 public class UserOrganizationRegistry : IUserOrganizationRegistry
@@ -32,7 +33,21 @@ public UserOrganizationRegistry(IUser user, IOrganizationRegistry organizationRe
             return null;
         }
 
-        return await _organizationRegistry.GetOrgShortName(orgNumber, cancellationToken);
+        var orgInfo = await _organizationRegistry.GetOrgInfo(orgNumber, cancellationToken);
+
+        return orgInfo?.ShortName;
+    }
+
+    public async Task<IList<OrganizationLongName>?> GetCurrentUserOrgLongNames(CancellationToken cancellationToken)
+    {
+        if (!_user.TryGetOrgNumber(out var orgNumber))
+        {
+            return null;
+        }
+
+        var orgInfo = await _organizationRegistry.GetOrgInfo(orgNumber, cancellationToken);
+
+        return orgInfo?.LongNames.ToArray();
     }
 }
 
@@ -41,4 +56,6 @@ internal sealed class LocalDevelopmentUserOrganizationRegistryDecorator : IUserO
     public LocalDevelopmentUserOrganizationRegistryDecorator(IUserOrganizationRegistry _) { }
 
     public Task<string?> GetCurrentUserOrgShortName(CancellationToken cancellationToken) => Task.FromResult("digdir")!;
+    public Task<IList<OrganizationLongName>?> GetCurrentUserOrgLongNames(CancellationToken cancellationToken) =>
+        Task.FromResult<IList<OrganizationLongName>?>(new[] { new OrganizationLongName { LongName = "Digdir", Language = "nb" } });
 }
diff --git a/src/Digdir.Domain.Dialogporten.Application/Common/Services/DialogActivityService.cs b/src/Digdir.Domain.Dialogporten.Application/Common/Services/DialogActivityService.cs
new file mode 100644
index 000000000..21dd4eb32
--- /dev/null
+++ b/src/Digdir.Domain.Dialogporten.Application/Common/Services/DialogActivityService.cs
@@ -0,0 +1,35 @@
+using Digdir.Domain.Dialogporten.Domain.Dialogs.Entities.Activities;
+using Digdir.Domain.Dialogporten.Domain.Localizations;
+
+namespace Digdir.Domain.Dialogporten.Application.Common.Services;
+
+public interface IDialogActivityService
+{
+    Task EnsurePerformedByIsSetForActivities(IEnumerable<DialogActivity> activities, CancellationToken cancellationToken);
+}
+
+public class DialogActivityService : IDialogActivityService
+{
+    private readonly IUserOrganizationRegistry _userOrganizationRegistry;
+
+    public DialogActivityService(
+        IUserOrganizationRegistry userOrganizationRegistry
+    )
+    {
+        _userOrganizationRegistry = userOrganizationRegistry;
+    }
+
+    public async Task EnsurePerformedByIsSetForActivities(IEnumerable<DialogActivity> activities, CancellationToken cancellationToken)
+    {
+        // TODO: if organization cannot be found we need to handle this. Put on a queue to be retried later(?) https://github.com/digdir/dialogporten/issues/639
+        foreach (var activity in activities)
+        {
+            var organizationLongNames = await _userOrganizationRegistry.GetCurrentUserOrgLongNames(cancellationToken);
+            activity.PerformedBy ??= new DialogActivityPerformedBy
+            {
+                Localizations = organizationLongNames?.Select(x => new Localization { Value = x.LongName, CultureCode = x.Language }).ToList() ?? new List<Localization>(),
+            };
+        }
+    }
+}
+
diff --git a/src/Digdir.Domain.Dialogporten.Application/Externals/IOrganizationRegistry.cs b/src/Digdir.Domain.Dialogporten.Application/Externals/IOrganizationRegistry.cs
index 9be3196e1..923f61760 100644
--- a/src/Digdir.Domain.Dialogporten.Application/Externals/IOrganizationRegistry.cs
+++ b/src/Digdir.Domain.Dialogporten.Application/Externals/IOrganizationRegistry.cs
@@ -2,5 +2,18 @@ namespace Digdir.Domain.Dialogporten.Application.Externals;
 
 public interface IOrganizationRegistry
 {
-    Task<string?> GetOrgShortName(string orgNumber, CancellationToken cancellationToken);
+    Task<OrganizationInfo?> GetOrgInfo(string orgNumber, CancellationToken cancellationToken);
 }
+
+public sealed class OrganizationLongName
+{
+    public required string LongName { get; init; }
+    public required string Language { get; init; }
+}
+
+public sealed class OrganizationInfo
+{
+    public required string OrgNumber { get; init; }
+    public required string ShortName { get; init; }
+    public required IList<OrganizationLongName> LongNames { get; init; }
+}
\ No newline at end of file
diff --git a/src/Digdir.Domain.Dialogporten.Application/Features/V1/ServiceOwner/Dialogs/Commands/Create/CreateDialogCommand.cs b/src/Digdir.Domain.Dialogporten.Application/Features/V1/ServiceOwner/Dialogs/Commands/Create/CreateDialogCommand.cs
index c2bf775f5..c8de6de81 100644
--- a/src/Digdir.Domain.Dialogporten.Application/Features/V1/ServiceOwner/Dialogs/Commands/Create/CreateDialogCommand.cs
+++ b/src/Digdir.Domain.Dialogporten.Application/Features/V1/ServiceOwner/Dialogs/Commands/Create/CreateDialogCommand.cs
@@ -2,11 +2,13 @@
 using AutoMapper;
 using Digdir.Domain.Dialogporten.Application.Common;
 using Digdir.Domain.Dialogporten.Application.Common.ReturnTypes;
+using Digdir.Domain.Dialogporten.Application.Common.Services;
 using Digdir.Domain.Dialogporten.Application.Externals;
 using Digdir.Domain.Dialogporten.Domain.Common;
 using Digdir.Domain.Dialogporten.Domain.Dialogs.Entities;
 using Digdir.Domain.Dialogporten.Domain.Dialogs.Entities.Activities;
 using Digdir.Domain.Dialogporten.Domain.Dialogs.Entities.Elements;
+using Digdir.Domain.Dialogporten.Domain.Localizations;
 using MediatR;
 using OneOf;
 using OneOf.Types;
@@ -26,6 +28,7 @@ internal sealed class CreateDialogCommandHandler : IRequestHandler<CreateDialogC
     private readonly IDomainContext _domainContext;
     private readonly IUserResourceRegistry _userResourceRegistry;
     private readonly IUserOrganizationRegistry _userOrganizationRegistry;
+    private readonly IDialogActivityService _dialogActivityService;
 
     public CreateDialogCommandHandler(
         IDialogDbContext db,
@@ -33,7 +36,8 @@ public CreateDialogCommandHandler(
         IUnitOfWork unitOfWork,
         IDomainContext domainContext,
         IUserResourceRegistry userResourceRegistry,
-        IUserOrganizationRegistry userOrganizationRegistry)
+        IUserOrganizationRegistry userOrganizationRegistry,
+        IDialogActivityService dialogActivityService)
     {
         _db = db ?? throw new ArgumentNullException(nameof(db));
         _mapper = mapper ?? throw new ArgumentNullException(nameof(mapper));
@@ -41,6 +45,7 @@ public CreateDialogCommandHandler(
         _domainContext = domainContext ?? throw new ArgumentNullException(nameof(domainContext));
         _userResourceRegistry = userResourceRegistry ?? throw new ArgumentNullException(nameof(userResourceRegistry));
         _userOrganizationRegistry = userOrganizationRegistry ?? throw new ArgumentNullException(nameof(userOrganizationRegistry));
+        _dialogActivityService = dialogActivityService ?? throw new ArgumentNullException(nameof(dialogActivityService));
     }
 
     public async Task<CreateDialogResult> Handle(CreateDialogCommand request, CancellationToken cancellationToken)
@@ -77,6 +82,8 @@ public async Task<CreateDialogResult> Handle(CreateDialogCommand request, Cancel
             _domainContext.AddError(DomainFailure.EntityExists<DialogElement>(existingElementIds));
         }
 
+        await _dialogActivityService.EnsurePerformedByIsSetForActivities(dialog.Activities, cancellationToken);
+
         await _db.Dialogs.AddAsync(dialog, cancellationToken);
 
         var saveResult = await _unitOfWork.SaveChangesAsync(cancellationToken);
diff --git a/src/Digdir.Domain.Dialogporten.Application/Features/V1/ServiceOwner/Dialogs/Commands/Update/UpdateDialogCommand.cs b/src/Digdir.Domain.Dialogporten.Application/Features/V1/ServiceOwner/Dialogs/Commands/Update/UpdateDialogCommand.cs
index 41f553d82..6c6e16e9a 100644
--- a/src/Digdir.Domain.Dialogporten.Application/Features/V1/ServiceOwner/Dialogs/Commands/Update/UpdateDialogCommand.cs
+++ b/src/Digdir.Domain.Dialogporten.Application/Features/V1/ServiceOwner/Dialogs/Commands/Update/UpdateDialogCommand.cs
@@ -2,12 +2,14 @@
 using Digdir.Domain.Dialogporten.Application.Common;
 using Digdir.Domain.Dialogporten.Application.Common.Extensions.Enumerables;
 using Digdir.Domain.Dialogporten.Application.Common.ReturnTypes;
+using Digdir.Domain.Dialogporten.Application.Common.Services;
 using Digdir.Domain.Dialogporten.Application.Externals;
 using Digdir.Domain.Dialogporten.Domain.Dialogs.Entities;
 using Digdir.Domain.Dialogporten.Domain.Dialogs.Entities.Actions;
 using Digdir.Domain.Dialogporten.Domain.Dialogs.Entities.Activities;
 using Digdir.Domain.Dialogporten.Domain.Dialogs.Entities.Content;
 using Digdir.Domain.Dialogporten.Domain.Dialogs.Entities.Elements;
+using Digdir.Domain.Dialogporten.Domain.Localizations;
 using MediatR;
 using Microsoft.EntityFrameworkCore;
 using OneOf;
@@ -32,19 +34,22 @@ internal sealed class UpdateDialogCommandHandler : IRequestHandler<UpdateDialogC
     private readonly IUnitOfWork _unitOfWork;
     private readonly IDomainContext _domainContext;
     private readonly IUserResourceRegistry _userResourceRegistry;
+    private readonly IDialogActivityService _dialogActivityService;
 
     public UpdateDialogCommandHandler(
         IDialogDbContext db,
         IMapper mapper,
         IUnitOfWork unitOfWork,
         IDomainContext domainContext,
-        IUserResourceRegistry userResourceRegistry)
+        IUserResourceRegistry userResourceRegistry,
+        IDialogActivityService dialogActivityService)
     {
         _db = db ?? throw new ArgumentNullException(nameof(db));
         _mapper = mapper ?? throw new ArgumentNullException(nameof(mapper));
         _unitOfWork = unitOfWork ?? throw new ArgumentNullException(nameof(unitOfWork));
         _domainContext = domainContext ?? throw new ArgumentNullException(nameof(domainContext));
         _userResourceRegistry = userResourceRegistry ?? throw new ArgumentNullException(nameof(userResourceRegistry));
+        _dialogActivityService = dialogActivityService ?? throw new ArgumentNullException(nameof(dialogActivityService));
     }
 
     public async Task<UpdateDialogResult> Handle(UpdateDialogCommand request, CancellationToken cancellationToken)
@@ -165,6 +170,9 @@ private async Task AppendActivity(DialogEntity dialog, UpdateDialogDto dto, Canc
     {
         var newDialogActivities = _mapper.Map<List<DialogActivity>>(dto.Activities);
 
+
+        await _dialogActivityService.EnsurePerformedByIsSetForActivities(newDialogActivities, cancellationToken);
+
         var existingIds = await _db.GetExistingIds(newDialogActivities, cancellationToken);
         if (existingIds.Count != 0)
         {
diff --git a/src/Digdir.Domain.Dialogporten.Infrastructure/Altinn/OrganizationRegistry/OrganizationRegistryClient.cs b/src/Digdir.Domain.Dialogporten.Infrastructure/Altinn/OrganizationRegistry/OrganizationRegistryClient.cs
index 131f6a322..a371f9dcf 100644
--- a/src/Digdir.Domain.Dialogporten.Infrastructure/Altinn/OrganizationRegistry/OrganizationRegistryClient.cs
+++ b/src/Digdir.Domain.Dialogporten.Infrastructure/Altinn/OrganizationRegistry/OrganizationRegistryClient.cs
@@ -8,7 +8,8 @@ namespace Digdir.Domain.Dialogporten.Infrastructure.Altinn.OrganizationRegistry;
 
 internal class OrganizationRegistryClient : IOrganizationRegistry
 {
-    private const string OrgShortNameReferenceCacheKey = "OrgShortNameReference";
+    private const string OrgNameReferenceCacheKey = "OrgNameReference";
+
     private static readonly DistributedCacheEntryOptions OneDayCacheDuration = new() { AbsoluteExpiration = DateTimeOffset.UtcNow.AddDays(1) };
     private static readonly DistributedCacheEntryOptions ZeroCacheDuration = new() { AbsoluteExpiration = DateTimeOffset.MinValue };
 
@@ -20,28 +21,35 @@ public OrganizationRegistryClient(HttpClient client, IFusionCacheProvider cacheP
         _client = client ?? throw new ArgumentNullException(nameof(client));
         _cache = cacheProvider.GetCache(nameof(OrganizationRegistry)) ?? throw new ArgumentNullException(nameof(cacheProvider));
     }
-    public async Task<string?> GetOrgShortName(string orgNumber, CancellationToken cancellationToken)
+
+    public async Task<OrganizationInfo?> GetOrgInfo(string orgNumber, CancellationToken cancellationToken)
     {
-        var orgShortNameByOrgNumber = await _cache.GetOrSetAsync(OrgShortNameReferenceCacheKey, async token => await GetOrgShortNameByOrgNumber(token), token: cancellationToken);
-        orgShortNameByOrgNumber.TryGetValue(orgNumber, out var orgShortName);
+        var orgInfoByOrgNumber = await _cache.GetOrSetAsync(OrgNameReferenceCacheKey, GetOrgInfo, token: cancellationToken);
+        orgInfoByOrgNumber.TryGetValue(orgNumber, out var orgInfo);
 
-        return orgShortName;
+        return orgInfo;
     }
 
-    private async Task<Dictionary<string, string>> GetOrgShortNameByOrgNumber(CancellationToken cancellationToken)
+    private async Task<Dictionary<string, OrganizationInfo>> GetOrgInfo(CancellationToken cancellationToken)
     {
         const string searchEndpoint = "orgs/altinn-orgs.json";
         var response = await _client
             .GetFromJsonAsync<OrganizationRegistryResponse>(searchEndpoint, cancellationToken) ?? throw new UnreachableException();
 
-        var orgShortNameByOrgNumber = response
+        var orgInfoByOrgNumber = response
             .Orgs
-            .ToDictionary(
-                pair => pair.Value.Orgnr,
-                pair => pair.Key
-            );
+            .ToDictionary(pair => pair.Value.Orgnr, pair => new OrganizationInfo
+            {
+                OrgNumber = pair.Value.Orgnr,
+                ShortName = pair.Key,
+                LongNames = pair.Value.Name?.Select(name => new OrganizationLongName
+                {
+                    LongName = name.Value,
+                    Language = name.Key
+                }).ToList() ?? new List<OrganizationLongName>()
+            });
 
-        return orgShortNameByOrgNumber;
+        return orgInfoByOrgNumber;
     }
 
     private sealed class OrganizationRegistryResponse
@@ -57,4 +65,4 @@ private sealed class OrganizationDetails
         public string? Homepage { get; init; }
         public IList<string>? Environments { get; init; }
     }
-}
+}
\ No newline at end of file
diff --git a/tests/Digdir.Domain.Dialogporten.Application.Integration.Tests/Common/DialogApplication.cs b/tests/Digdir.Domain.Dialogporten.Application.Integration.Tests/Common/DialogApplication.cs
index b8e7a7483..3edc304f9 100644
--- a/tests/Digdir.Domain.Dialogporten.Application.Integration.Tests/Common/DialogApplication.cs
+++ b/tests/Digdir.Domain.Dialogporten.Application.Integration.Tests/Common/DialogApplication.cs
@@ -118,8 +118,13 @@ private static IOrganizationRegistry CreateOrganizationRegistrySubstitute()
         var organizationRegistrySubstitute = Substitute.For<IOrganizationRegistry>();
 
         organizationRegistrySubstitute
-            .GetOrgShortName(Arg.Any<string>(), Arg.Any<CancellationToken>())
-            .Returns("digdir");
+            .GetOrgInfo(Arg.Any<string>(), Arg.Any<CancellationToken>())
+            .Returns(new OrganizationInfo
+            {
+                OrgNumber = "991825827",
+                ShortName = "digdir",
+                LongNames = new[] { new OrganizationLongName { LongName = "Digitaliseringsdirektoratet", Language = "nb" } }
+            });
 
         return organizationRegistrySubstitute;
     }