Slack allows you and your team to collaborate more easily through an organized messaging system. The Slack web application is available in Shift for any Shift Advanced user.
Click on Settings (the wrench in the bottom left-hand corner)
Click on the Apps heading
Search for Slack
Name your account, choose your colour, and enter your team domain
Click on the Slack icon in the Shift toolbar and log in
Use the same steps as above to add another Slack account, then log in with your other workspace URL.
We have a more detailed article on the process here.