I think the profiles are supposed to be generated when you sign into the app. Lots of people have posted questions about this on Reddit and the Comcast Support forum with varying success. Some were able to use older versions of the app or get it working by either:
going to Settings > Network & Internet > Internet > Reset internet (button in top right corner)
going to Settings > System > Reset Options > Reset Wi-Fi, mobile & Bluetooth
Not everyone had success with that, and they were using stock Android. I haven't found anything about using these hotspots with Graphene.
I assume the first thing you tried was going into Internet settings, hitting the gear icon next to the xfinitywifi network, and ensuring auto-connect was enabled. If not then check that setting.
I should be within range of a xfinitywifi hotspot tomorrow, I'll try to remember to test it and report back.