https://openhive.chat/ is the open public chat where you can follow the witness channel, channels here are bridged from mattermost.
https://chat.hive.blue/hive is the mattermost (slack like chat rooms) it's invite only afaik, simply message guiltyparties for an invite to get in I think.

The main hive discord server also has a witness-info channel which I guess can also be used for questions.