If you have just a few users, Rocket is very easy to install and has always worked well on a $5 DO instance. My preference is for Zulip, as it's easier to track multiple discussions, but I don't have an incentive to switch now that my Rocket instance is running.

I have no experience with more than a few users. If you want a hosted solution, I have seen reports that Zulip works well with a large number of users.