They are using a guest side on XFinity customers routers to allow other customers access to the Internet. They say it doesn't affect bandwidth of the "host" customer (I don't see how this is true), and doesn't affect their data quota. You have to opt out of the program.
I'm not an xfinity customer, but I'd certainly opt out. The average ISP provided equipment generally isn't known for getting updated, who knows if there is a security vulnerability that allows someone to get to the other side?