If that's the entire problem, then why not make these forums read-only by default (while still keeping them publicly viewable) and hand out invite links as in D

If that's the entire problem, then why not make these forums read-only by default (while still keeping them publicly viewable) and hand out invite links as in Discord? Lobste.rs, for instance, has an interesting "invite tree" concept where your reputation is bound to people you've invited and who they've invited and so on.