Could lemmy be used as a private self hosted forum?
And could lemmy be used as a private forum were only registered users and whitelisted users can view, post and comment on the instance. This way you can have a private instance which only paid users of your group or club can join like many membership website forums have.
There’s a phpbb forum front end you can use. Having it private yet federated doesn’t make any sense though, but yes you can use it completely on its own with federation turned off.