Can you self-host, or are you looking for another online service? Facebook Groups is basically a forum when it comes down to it, and any forum software can do what you’re asking. I really like Discourse. You can self-host it for free (well, whatever your server costs), they’ll host it for free if you’re an open-source project, or if you’re a legal non-profit you can get 50% off their hosting for $25-50/month.