From OStatus Community Group
Groups have Atom feeds just like users do and this of course means that groups can be run over OStatus too!
As a user on Yoursite.tld you can join a group on Example.net without setting up an Example.net account. Group posts from Example.net will appear right in the joined user's home timeline on Yoursite.tld just like any local group. Then it is possible to post back and forth to the group.