Communities is where your group talks between sessions: questions, session notes and project reviews.
Questions get answered
Ask something and people answer. Threads stay readable because the useful replies rise, rather than the loudest ones.
Moderation is built in
Banned words, blocked links, slow mode and rate limits all exist, so a group stays usable long after one person could read everything in it.
Follow people
Follows, mentions and notifications work. You hear about the replies that matter to you and not about everything else.