Smith is correct. I have HBO GO on all of my devices and my TV and I have Xfinity/Comcast the same as everyone else.
Make sure when you sign in not to click on *Existing User* even though you already have an account with Comcast. When signing into HBO GO, first go online on your computer to get a HBO GO *ID* and when you sign in on your tablets and smartphones, click on *NEW USER*.
Cyn