Most forums have private boards or even private discords etc that you’d never know about until invited. (And if you do know about them they’re not the private ones, just the restricted ones - many layers like onions.)

I can't link either of the two I use here for obvious reasons, but the primary one I frequent is just a private website made by a bunch of Googlers. It's basically just an invite only forum with a more complicated trust/permission system. It's very active with roughly 5,000 users and access to boards is limited by a trust system which operates similarly to private torrent trackers.

The second one, which I only use occasionally, is a private Android/iOS app that is very similar to Signal group chats.

I've heard of quite a few others but I don't have first hand experience with them.