@emacsen
Not yet, but soon. As in later this year
I'll have group actors in Sputnik Opphuichi and message routing with envelope rewriting in the queue software
Compatibility with current services is the difficult part. Sending and receiving messages isn't a problem, but Mastodon and Pleroma users won't be able to manage their subscriptions in ActivityPub. They'll need a fallback in the form of a web interface or Majordomo commands
@emacsen
My opinion on ActivityPub extensions is that ActivityPub C2S is Turing Complete and therefore sufficiently expressive for any computation, so the best way to add functionality in the ActivityPub ecosystem would be to promote end to end ActivityPub, implement new features with existing vocabulary, and publish extensions to make implementation easier once the feature is mature
Some distribution patterns can be implemented with Follow*, but other patterns and administration will require additional Collections
Add and Delete will work with a Subscriber Collection just add well as a Subscribe Activity would
I should do an architecture document. There are a lot of expectations inherited from mailing lists and I think it's important to reproduce the desirable features and incorporate the lessons from 30+ years of email history