IMO Discord is the best platform for this right now, which is unfortunate. The little I’ve tried Matrix has not been very impressive (single chatrooms, slow, bad self-hosting experience IMO), IRC is a bit better (though very dated in many regards, esp. user management) but still doesn’t have the categories/channels that make discord nice. And most other chats are proprietary with discord just being the best one.
Which one would you like them to use?
It’s fairly new I think. I ran into it first time a week or two ago when going into a test account I haven’t used for a while.
Shame really, having at least two users is very useful when building bots. Testing user-specific interactions and such.