When I sign in and go to the Users and Preferences page, there is supposed to be a tab called Add a User. But no such tab appears. How can I add a second user to the account?
There should be the 'Add a User' tab. If you don't see it, contact Comcast to get the issue sorted. Sounds like you should be able to add the user from the location you are at. Comcast should be able to add another user for you if you call them.