Although you can’t see your contact list when Sortd is in Fullscreen mode you will still receive messages if someone sends you a chat on Hangouts (the screen will pop up over Sortd), similarly if you leave a Hangout chat open. 

You can just minimise Sortd or put it into Sidebar mode to see your contacts. 

Alternatively you can install the Google Hangouts Chrome Extension or App for Hangouts.

Chrome Extension

App (Only works on Windows and Chrome OS)