Check out Contact Grabber at
Class: Contact Grabber (msn, orkut, contact, gmail) - PHP Classes
"The class will fetch the contacts of the selected site for the given username and password. Yahoo, Gmail & MSN Messenger classes have been incorporated as they are available under GPL. Rediff, Orkut & MySpace classes have been developed at Magnet.
AOL & Lycos have been incorporated as they were also available under GPL. New classes for LinkedIn & Lycos have been integrated."
This could be made into a Pligg module. I don't have time to do it right now, but it shouldn't be that difficult to add this function to the Invite a Friend module.