Your question is not clear.
If you want to create a new user ID (email address), log in to https://customer.xfinity.com/#/users, then scroll down to and click "Add a New User".
If you want to add an existing external email address to your Comcast webmail page, see the "external account" links under "Email Account" on https://connect.xfinity.com/faq/email.html.
If you want to do something else, what it is?
