How to login into Drupal using Google Apps username and password?
To integrate your Drupal 7 site with Google Apps, you need to:


Enable the OpenID module on your drupal site
Enable the Federated Login Service using your Google Apps control panel at http://www.google.com/a/cpanel/yourdomain.com/SetupIdp
Add an X-XRDS-Location header at yourdomain.com/openid  For example, if yourdomain.com is a PHP site, upload this as /openid/index.php

&lt;?php
header('X-XRDS-Location: https://www.google.com/accounts/o8/site-xrds?ns=2&amp;hd=yourdomain.com');
?&gt;

Go to your drupal site's login page. You'll see a new 'login with open id' link. Click it. Enter: https://google.com/accounts/o8/site-xrds?hd=yourdomain.com and click submit
The first time you do this, you'll be asked if you want to allow your site to authenticate. Click yes.
Subsequently, if you try to login using the above URL and you are logged in to Google Apps, you'll be logged in directly.