How to IRC
We self-host mattermost now, it's easier to use if you're not familiar with IRC. If you know how to use IRC, we're #csc on freenode and there should be a #csc bridge that relays messages between #csc and our Mattermost instance.
Both #csc on Freenode (Freenode Webchat or via IRC clients e.g. irssi, weechat) and ~freenode-csc on Mattermost are official channels to interact with CSC members!
Make an account at our self-hosted mattermost. For username, you can put your questid (i.e. your CSC username), although you can always set your full name as it will appear in mattermost.
The benefit of mattermost over slack and family is that slack stores all your information on slack's servers, wherever they are in the US. They do this so they can sell your data back to you (e.g. not allowing you to see old messages), but slack is also closed-source even though it was derived from IRC. Mattermost is open-source and hosted on CSC servers.