If you need to formally require users to sign-in with a google account....

Start your functions.php off with this line:
require_once $_SERVER['DOCUMENT_ROOT'].'/google-auth/api.php';

This provides access to the following functions:

• google_isLoggedIn() //RETURNS TRUE/FALSE
• google_getUserEmail() //RETURNS STRING (NULL IF NOT LOGGED IN)
• google_sendToAuth($redir="new/target/webpage/after/auth.html") (prompts the user through google auth)
• google_logout() //REMOVES ALL LOGIN INFORMATION

• Simple usage example
• Want to debug via different email addresses?