@astrid My idea of publically-readable groups that are compatible with the current twitter-like model:
Group account has a special root post & it boosts all its direct replies. The fact that it is a reply will hide it from members' non-reply outbox view.
The fact of accepted membership should be encoded either as accepted follow request or as a mutual follow.
Restricted readability would be harder. I think, the group should send update activity that changes recipients of the private root post. I am not sure how other servers would react, but I think group's outbox should contain direct private replies as boosted and patch it's recipient list to match it's current members.
