feat: add group user by id restriction

Adding by ID can only be done for existing org members

test/test_iam.py

@@ -4,7 +4,7 @@ import pytest
 from httpx import AsyncClient
 
 from src.user.models import User
-from src.organisation.models import Organisation as Org
+from src.organisation.models import Organisation as Org, OrgUsers
 from src.iam.models import Group
 
 from .conftest import generate_query_and_status
@@ -468,6 +468,8 @@ async def test_put_group_user_success(default_client: AsyncClient, db_session):
 		)
 	)
 	db_session.flush()
+	db_session.add(OrgUsers(user_id=2, org_id=1))
+	db_session.flush()
 
 	resp = await default_client.put(
 		"/iam/group/user", json={"user_id": 2, "group_id": 1, "organisation_id": 1}