A lot depends on your goal.  For a group/community, is it about having a private conversation with a group of people that you know or is it about keeping the conversation on a subject, so your feed isn't full of subjects that don't interest you at the moment or at all.  Limiting the subject matter would likely be done completely differently because you are happy for new people to join in the conversation.  If you want only your desired group to talk, regardless of the subject, it would need to work much differently.  If you needed it to be perfectly secure, then it gets way more complex.