We haven't implemented federation of Groups yet, but group tags and discussion on how unbounded groups can be federated via ActivityPub has already started.
Concerning federation of traditional groups, we will port our logic from v2 and translate following the same rules as AndStatus because they work, as yvolk beautifully stated :)
Finally, also note that the group actors now have "self-tags", as the other actors, which was a milestone.
gnusocial