Hi,
What you are suggesting - using Google Accounts as a registration
mechanism for a website, is not something that the current
authentication mechanisms are meant to do.
Since you would not want to handle credentials directly, you would
have to use AuthSub. AuthSub only gives you back a token (not a user
ID of any sort) and this token changes with each sign in, so you can't
track users directly with it. Furthermore, to use AuthSub you have to
request access to some kind of private user data (like access to their
calendar) in order to retrieve a token. There is no scope to request
only authenticating the user or to get back just their e-mail address
or something equivalently unique and useful.
This is a limitation of what the authentication system was intended to
be used for, though we are aware of the requests to make it more
abstract.
Cheers,
-Jeff