The only alternative I can think of that all of your community probably already have is email. Set up an email mailing list and lock it down so only members of your community and those invited by your community can use it. Despite the wishes of some here email will never go away and will be used by anyone communicating with businesses.

For your bonus criteria email can be self hosted but that's a more complicated topic as it pertains to mailing lists. At least a couple people in your community should be at list technical enough to follow internet examples. Mailing lists are federated per the spirit of the definition as they can each use their own existing email provider.