If I go to the “Test your AuthSub registration” page, it asks if my website can access my calendar. But what I want is only to “Sign in using your Google account”.
How do I tell Google that I don’t want to access someone’s calendar, but only want them to sign in using their Google account?
You should not be looking into oAuth, it is an authentication protocol while exchanging data with google services using APIs, for signing in using google account you should look into “Federated Login for Google Account Users”
Please read the following article.
http://code.google.com/apis/accounts/docs/OpenID.html