You must be invited via email to register with the service. To set up your sharing account you will need to create a password. You will also have the option to register with Google Authenticator on your phone.
Using the service
Once you receive an email and click on a Comcast document sharing link you will be prompted to enter your password, plus a Google Authenticator or SMS verification code, before you can access the document.
If you have problems activating your account or if you need to reset your password click on the Account Recovery link and follow the included instructions.