Invitation software for social networking platform

We're looking for specific functionality in our social networking website. When users sign up, we want to offer checking their google, yahoo, hotmail address book for a) users currently in the system b) automatically connect with friends currently in the system c) send invitations to those not in the system to join. Can you give a recommendation for how to add this functionality? (prefer to outsource rather than build in house)

Thanks in advance.