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.
[1] http://www.mattermost.org/