I host Rocket.Chat for my wife’s business. The free self-hosted version now allows up to 50 users before having to purchase a license (was 25 prior to the version 7 update). If you kick a user you simply deactivate their account and regain the user slot. Rather than deleting a user completely which would additionally delete any messages they’ve posted.

Easy to host, easy to upgrade. Mobile apps both IOS and Android work well.

I wanted to use Zulip as I felt the interface was cleaner. But the 10 user limit was a deal breaker.

That’s not what the other poster said. He said “Non Featured” servers. You have to do a DNS hack on the switch to connect to a private server.