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.