Signups have been closed for 2.5 years since they were acquired by Slack.

http://blog.screenhero.com/post/109337923751/screenhero-join...

Yep. That's what I was talking about.

invites seem to work still

I think he means that since Slack bought them, they closed the registrations and have been "integrating with Slack" for some over a year now I belive.