Over in another thread, Steve said something that stuck with me:
I think a Collection could have an inbox, without becoming an Actor, and that kinda makes both options defensible logically, although I can see pros and cons to both. “Collection with an inbox” seems to me appropriate for collections objects that emphatically aren’t Actors (say, a subreddit, a channel, a thing Actors post into), while “Actor Group” (the multi-type enthusiasts might even want to insist on the "type": {"Group", "Actor"}" rather than just refining Group as an extension of Actor!) seems more appropriate for a group chat, a mailing list, a group of Actors who act collectively and can be blocklisted or moderated as if they functioned as one Actor.
Basically I’m worried our fear of neologism is leading us to cram disparate things into the same data model to avoid writing a FEP with an @Context extension-- maybe Groups and Collections both deserve to exist for different use-cases! 