https://chat.18f.gov/?channel=devops-public, actually. EDIT: Gaaah, sorry, down right now for reasons. Will post here again when it's back up. Sorry!
Watchdog: 18F's Slack security exposed GSA data https://fcw.com/articles/2016/05/13/slack-security-18f.aspx
It was enough of a problem that the reaction was to probably take it down and take a hard look at everything before being it back up. Maybe a hosted rocket.chat will be the replacement to allow them more granular control over security.
Seems like the Slack invite app is not deployed to cloud.gov right now.