For what teams focuses on, matrix. Has channels, big voice calls, etc. missing the sheer set of plugins teams has.

Xmpp/jabber is also good but since it started as a classic instant messaging protocol it’s facilities for persistent storage and groups is a bit wonky.

For friend groups you’re better off with signal as it has enough administration tools to manage a small group and almost no overhead for the people you want to join you (although they will whine and drag their feet like petulant children because you made them download an app).