Addressbook Importer Script / Invite Script, like this one
[url removed, login to view]
Openinviter ([url removed, login to view]) provides such service.
Caution: the backend code has to be written in codeigniter
With users' login permission, it needs to be able to look into different email providers and retrieve users' contact list info (mainly their friends' names and email addresses), then depending on need it can send emails to users' friends.
For email providers, we need GMAIL, YAHOO, WINDOWS LIVE/HOTMAIL, AOL, AIM... just the major ones only
Please show demo when you contact me.