If you see today successful websites like Hi5, StumbleUPon or facebook etc.

They all have feature that users can access to their address books of hotmail, yahoo or aol,  then see which friends are using the site and invite them directly without opening their mail box.

It's the most cheapest way to advertise your site users to users. One user invite their 10 friends those 10 invited also send another 10 each, see in one hour you site will be seeing by millions of user around the world. I am happy to spend to buy that kind feature for my site rather than spending money on google adwords. It's the most effective way for your campaign.

Please vld if you can add this feature, I am sure everybody happy to pay money for this feature or is there anyone who can do that?