Login to ASP.net application via Google account

Use OpenID. This is implemented in ASP.NET in the DotNetOpenAuth library. It includes samples for Google logins, and it's very easy.

The roles should work to. As you build up your user table, just add users to the appropriate roles.

Have a look at this:

http://www.asp.net/vnext/overview/aspnet/oauth-in-the-default-aspnet-45-templates

Scott Hanselman giving a quick overview of the OpenID features in .NET4.5. This might be a good starting point for you.

On MSDN: http://blogs.msdn.com/b/webdev/archive/2012/08/15/oauth-openid-support-for-webforms-mvc-and-webpages.aspx

I suggest you to take a look at this codeplex miniproject : http://loginwithgoogle.codeplex.com/. You might find it helpful.

Regards