You can use your account number or Social Security Number to get your Comcast ID (username).
About Comcast ID
Each account number has a primary Comcast ID. A Comcast ID is a username, and has a password with a secret question and answer for password recovery. If you're an XFINITY Internet customer, your primary Comcast ID was created when you activated your Internet service.
If you forget your Comcast ID, you can look it up. You'll need to provide your account number or your date of birth and the last four digits of your Social Security Number. This allows us to verify your identity without your account number.
If you're an XFINITY Internet customer and you look up your Comcast ID on your home computer, you'll only need to enter the phone number associated with your account.