We offer weekly deals in our website. We would like to engage a social experience by offering bigger discounts if a user invites 3 his/her friends to the site. I wonder if there are special services which offer such functionality to integrate to your site.. We could implement it, but we prefer to safe some time and pay for such service if it exists. Thanks.
I would recommend making a referral system with PHP using a MySQL back-end. These are relatively simple to make so the cost of developing one should be fairly low. Do a Google search for a 'PHP Referral/Invite System' and see what you can find.