diff --git a/lib/federation/activity_pub/transmogrifier.ex b/lib/federation/activity_pub/transmogrifier.ex
index a8b044371..95f535702 100644
--- a/lib/federation/activity_pub/transmogrifier.ex
+++ b/lib/federation/activity_pub/transmogrifier.ex
@@ -837,7 +837,7 @@ defmodule Mobilizon.Federation.ActivityPub.Transmogrifier do
 
   # Handle incoming `Accept` activities wrapping a `Join` activity on an event
   defp do_handle_incoming_accept_join(join_object, %Actor{} = actor_accepting) do
-    case get_participant(join_object) do
+    case get_participant(join_object, actor_accepting) do
       {:ok, participant} ->
         do_handle_incoming_accept_join_event(participant, actor_accepting)
 
@@ -868,9 +868,9 @@ defmodule Mobilizon.Federation.ActivityPub.Transmogrifier do
          %Actor{} = actor_accepting
        )
        when role in [:not_approved, :rejected] do
-    # TODO: The actor that accepts the Join activity may another one that the event organizer ?
-    # Or maybe for groups it's the group that sends the Accept activity
-    with {:same_actor, true} <- {:same_actor, actor_accepting.id == event.organizer_actor_id},
+    with %Event{} = event <- Events.get_event_with_preload!(event.id),
+         {:can_accept_event_join, true} <-
+           {:can_accept_event_join, can_accept_event_join?(actor_accepting, event)},
          {:ok, %Activity{} = activity, %Participant{role: :participant} = participant} <-
            ActivityPub.accept(
              :join,
@@ -881,8 +881,8 @@ defmodule Mobilizon.Federation.ActivityPub.Transmogrifier do
            Participation.send_emails_to_local_user(participant) do
       {:ok, activity, participant}
     else
-      {:same_actor} ->
-        {:error, "Actor who accepted the join wasn't the event organizer. Quite odd."}
+      {:can_accept_event_join, false} ->
+        {:error, "Actor who accepted the join didn't have permission to do so."}
 
       {:ok, %Participant{role: :participant} = _follow} ->
         {:error, "Participant"}
@@ -917,7 +917,7 @@ defmodule Mobilizon.Federation.ActivityPub.Transmogrifier do
   defp do_handle_incoming_reject_join(join_object, %Actor{} = actor_accepting) do
     with {:join_event, {:ok, %Participant{event: event, role: role} = participant}}
          when role != :rejected <-
-           {:join_event, get_participant(join_object)},
+           {:join_event, get_participant(join_object, actor_accepting)},
          # TODO: The actor that accepts the Join activity may another one that the event organizer ?
          # Or maybe for groups it's the group that sends the Accept activity
          {:same_actor, true} <- {:same_actor, actor_accepting.id == event.organizer_actor_id},
@@ -1025,14 +1025,22 @@ defmodule Mobilizon.Federation.ActivityPub.Transmogrifier do
     end
   end
 
-  defp get_participant(join_object) do
+  defp get_participant(join_object, %Actor{} = actor_accepting, loop \\ 1) do
     with join_object_id when not is_nil(join_object_id) <- Utils.get_url(join_object),
          {:not_found, %Participant{} = participant} <-
            {:not_found, Events.get_participant_by_url(join_object_id)} do
       {:ok, participant}
     else
       {:not_found, _err} ->
-        {:error, "Participant URL not found"}
+        with true <- is_map(join_object),
+             true <- loop < 2,
+             true <- Utils.are_same_origin?(actor_accepting.url, join_object["id"]),
+             {:ok, _activity, %Participant{url: participant_url}} <- handle_incoming(join_object) do
+          get_participant(participant_url, actor_accepting, 2)
+        else
+          _ ->
+            {:error, "Participant URL not found"}
+        end
 
       _ ->
         {:error, "ActivityPub ID not found in Accept Join object"}
@@ -1129,4 +1137,22 @@ defmodule Mobilizon.Federation.ActivityPub.Transmogrifier do
       _ -> {:error, :remove_object_not_found}
     end
   end
+
+  defp can_accept_event_join?(
+         %Actor{url: actor_url} = actor,
+         %Event{attributed_to: %Actor{type: :Group, url: group_url} = _group} = event
+       ) do
+    actor_url == group_url || Permission.can_update_group_object?(actor, event)
+  end
+
+  defp can_accept_event_join?(
+         %Actor{id: actor_id},
+         %Event{organizer_actor: %Actor{id: organizer_actor_id}}
+       ) do
+    organizer_actor_id == actor_id
+  end
+
+  defp can_accept_event_join?(_actor, _event) do
+    false
+  end
 end
diff --git a/test/federation/activity_pub/transmogrifier/join_test.exs b/test/federation/activity_pub/transmogrifier/join_test.exs
index d26c4eb89..3948bf40e 100644
--- a/test/federation/activity_pub/transmogrifier/join_test.exs
+++ b/test/federation/activity_pub/transmogrifier/join_test.exs
@@ -64,6 +64,46 @@ defmodule Mobilizon.Federation.ActivityPub.Transmogrifier.JoinTest do
       # We don't accept already accepted Accept activities
       :error = Transmogrifier.handle_incoming(accept_data)
     end
+
+    test "it accepts Accept activities with an inline Join from same origin" do
+      %Actor{} = organizer = insert(:actor)
+
+      %Actor{url: participant_actor_url} =
+        insert(:actor, domain: "somewhere.else", url: "https://somewhere.else/@participant")
+
+      %Actor{} =
+        group = insert(:group, domain: "somewhere.else", url: "https://somewhere.else/@group")
+
+      insert(:member, actor: organizer, parent: group, role: :moderator)
+
+      %Actor{} =
+        actor_member_2 =
+        insert(:actor, domain: "somewhere.else", url: "https://somewhere.else/@member")
+
+      insert(:member, actor: actor_member_2, parent: group, role: :moderator)
+
+      %Event{url: event_url} =
+        insert(:event, organizer_actor: organizer, join_options: :restricted, attributed_to: group)
+
+      join_data =
+        File.read!("test/fixtures/mobilizon-join-activity.json")
+        |> Jason.decode!()
+        |> Map.put("actor", participant_actor_url)
+        |> Map.put("object", event_url)
+        |> Map.put("participationMessage", @join_message)
+        |> Map.put("id", "https://somewhere.else/@participant/join/event/1")
+
+      accept_data =
+        File.read!("test/fixtures/mastodon-accept-activity.json")
+        |> Jason.decode!()
+        |> Map.put("actor", actor_member_2.url)
+        |> Map.put("object", join_data)
+
+      {:ok, accept_activity, _} = Transmogrifier.handle_incoming(accept_data)
+      assert accept_activity.data["object"]["id"] == join_data["id"]
+      assert accept_activity.data["object"]["id"] =~ "/join/"
+      assert accept_activity.data["id"] =~ "/accept/join/"
+    end
   end
 
   describe "handle incoming reject join activities" do