(I'm a little ashamed that I lurk on this list but the thing that brings me out of lurking is a post on communication technologies)
Not Zulip.
Please not Zulip.
I've used Zulip once many years ago, and once this year. Both times, its interface felt clunky, and it felt like it was trying to get the best of IRC, Slack, and a BBS and instead getting the worst of all the worlds. Synchronous communication was confusing, asynchronous communication was limited, and topics and channels didn't really do anything to organize things.
I have two alternatives to add to consideration:
* Mattermost: it's an open-source clone of Slack, which has its advantages and disadvantages, but at least it's familiar and heavily used UI
* Discourse: it's an asynchronous communication tool, but that may acceptable or preferable, and there is already a Haskell Discourse set up (
https://discourse.haskell.org/)