On my website, Registered users should be able to refer the website to friends. Each registered user should have unique invitation code. For example, if a users user id is 29993 ( auto increment id from registration data base table) then his invitation link can be [url removed, login to view] New registrations from the invitation link should be tracked from the invitation link id and stored in registration database table.
User should be able to send email (from address book) through major Email service provider (Gmail, yahoo, facebook ONLY.). There should also be option for Manual invites (User has to input email Address)