I have recently implemented the SSO functionality for a Google Apps Marketplace app we are developing. In simple words: it provides a way to retrieve the Google Apps' user's email and log him in in your website, without the need of authorization on his end. You just need the consumer key and consumer secret, provided by Google to the app during installation on your domain (the installing user also authorizes (a one time action) any other permissions you request in the Manifest file).
Now I have somehow managed to get the SSO user login working using JanRain's OpenID PHP library and adding Google Apps as provider using the PHP Extensions for Google Apps OpenID Discovery.
However, after logging in, I need to implement a functionality that will retrieve all users in a given Google Apps domain. I've already did that using oAuth2 authentication and the following Directory API. However, this requires the existense of a consumer key, consumer secret and a redirect URL (that must be registered in the Google API console).
Is there a way to remove this convenience and instead allow our users to directly be able to get their Google Apps domain's users, using the existing SSO authentication we made in the background while logging him in? Otherwise, it will be too much hassle for the user to register the app at the Google API console, enter the correct redirect URL and set it up in our website and then he will be able to get his domain's users.
Regular users cannot use the Directory API, you'll need to authenticate as an admin user to make Directory API calls.
Depending on your needs though for accessing all users, you may be able to get by with requesting access to the user's Contacts scope and grabbing a copy of the full Global Address List which contains information on all non-hidden domain users as well as non-hidden groups and shared contacts.