Places that have Xfinity WiFi have things called "hotspots". To connect to them first you must have Xfinity WiFi. To know if there is a hotspot there, go to your WiFi settings on your device and if it shows "xfinitywifi" or "Xfinity" they have a hotspot there. To access it, click on either on and enter your Xfinity login credentials.
Please, let me know if this does not work.