There are many different types of such systems available on the internet. If you need it to get sign-ups for a website that will be launching, you can use http://launchrock.com. If you have a wordpress blog, you can use various wordpress plugins: http://wordpress.org/extend/plugins/tags/referral. There are many general rewards scripts you could use, e.g. http://www.hotscripts.com/listing/projektirs-referral-reward.... If you want to pay cash rewards, then you should use a general affiliate script. You can look at some examples here: http://www.hotscripts.com/category/scripts/php/scripts-progr...