I think that the hardest part is figuring out how to define "known users" without making it a big manual hassle that essentially breaks the usage.
Application form / New user approval
I'm looking into moving our gaming guild to nodebb but to do so I'm looking for a feature that is something like an approval system for new users.
Among the lines of: New user fills a form and existing users can ask questions so the new applicant can answer to those before he gets the approval to join.
If you know guildwork.com - it as this type of approval system - for those who want to take a look there you can create a new account without email verification to see how this could look like from the backend.
As a flow chart maybe: