As described by macgirvin on SocialHub: https://socialhub.activitypub.rocks/t/pixelfed-groups/1984/48
I think it should be possible to implement this while maintaining the current compatibility, and is probably the path that Smithereen and Pixelfed are taking for their groups
Current work is on the
new-group-federationbranch
now merged, hard to test without other implementations though
probably more to do in the future but for now I'll consider it done