Creating private chat groups is another matter. I honestly think Matrix chat is the best approach for secure and private group connectivity in the FediVerse. Though some might argue for Signal.
I was just passing through and thought I would throw in my two cents on this, as someone with a bit of experience. Session and SimpleX both work as ‘secure and private group chat’ platforms, as they are E2EE and require no phone numbers/other private information. SimpleX is a little more feature rich, but I’ve found Session more stable (although SimpleX is improving. They also lack the geek-curve @ExhibiCat@lemmynsfw.com mentioned, as you don’t have to do much more than pick a username to start chatting (unless you want to dig into all the technical settings). Depending on what you are looking for in a chat app, you may want to check them out.
e: fixed the link for SimpleX
I don’t think it is from one. The artist is lvl, if you are looking for more of their work.