If a users logs into this website using Google, this is what happens:
- We send a request to Google, asking them to verify you
- You must already be logged into Google at this point. If not, they will ask you to login.
- Once you're logged in, Google confirm your identity and provide us with your email address and name
- We use this to create your account on our website
This is a very basic setup. Some websites ask for more information than just your name and email address. And yes, some ask for much more!
However, Google clearly warn you about any requests for sensitive information (e.g. your location, or 'friends' in your 'circle'). You should get a message something like this:
This application / website wants to access your: Name, Email address and Friends List.
So you're normally kept informed about the information you're sharing.
But if you're still concerned about websites sharing your information, you will probably be happier signing up to each site individually using an email and password.