# UNDF: UNDF-2026-000000061
--- a/src/mod_shared_roster.erl
+++ b/src/mod_shared_roster.erl
@@ -351,10 +351,10 @@ process_subscription(Direction, User, Server, JID, _Type, Acc) ->
     {DisplayedGroups, _} = get_user_displayed_groups(US),
-    SRUsers = lists:usort(lists:flatmap(fun (Group) ->
-						get_group_users(LServer, Group)
-					end,
-					DisplayedGroups)),
-    case lists:member(US1, SRUsers) of
+    SRUsersSet = lists:foldl(
+        fun(Group, Acc0) ->
+            lists:foldl(fun(U, S) -> gb_sets:add(U, S) end,
+                        Acc0, get_group_users(LServer, Group))
+        end, gb_sets:empty(), DisplayedGroups),
+    case gb_sets:is_member(US1, SRUsersSet) of
       true ->
 	  case Direction of
 	    in -> {stop, false};
@@ -658,7 +658,8 @@ is_user_in_group(US, Group, Host) ->
     Mod = gen_mod:db_mod(Host, ?MODULE),
     case Mod:is_user_in_group(US, Group, Host) of
 	false ->
-	    lists:member(US, get_group_users(Host, Group));
+	    GroupSet = gb_sets:from_list(get_group_users(Host, Group)),
+	    gb_sets:is_member(US, GroupSet);
 	true ->
 	    true
     end.