Nusm

I think that not federating would be better for a private community, but maybe that's just me. If it federates, then it's not really private.

I don't know anything about the safety and security, I just wanted to point out that Piefed has had private communities for about 4 months now. One can be set up where the only way you can get in is being invited by a member, a mod, or the owner of the community.