Hey Jian, I am not sure about a specific "refer-a-friend" service, but you might save some money and get some other useful tools, by looking into one of the better autoresponders, to handle all of your emailing needs. Just ask people about their favorite autoresponder. I remember a couple of them include html code you can put on your site that will setup "refer-a-friend", newsletter subscribing or whatever.
Just a thought.
Mark Hopkins
http://mbiz77.167aday.com