If you are using a Comcast supplied gateway device, it does not have that feature. They expect you to use the secondary public WiFi hotspot (xfinitywifi) that is also broadcasted for your guests.
I think that's great news. Here is my next question: What is the password for that secondary WiFi hotspot; what kind of limits does it have, and why were we never informed of it when we opened our account? We keep seeing that on our list of available networks, but thought it was from a neighbor or something - we had no idea it was our own!
We also had a technician working on WiFi strength issues and our concrete walls, who told us the Guest Network was available. How does this happen, that they don't even know their own product?