Referring people to survey and PTC sites

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I realize that much of the money that you make on PTC or survey sites can be done by getting referrals. Can anyone tell me how to do that? What I mean is, once you have someone that wants to join the site, how does the site know that I referred them? Do they use your user name or another code when registering? Or, when I am trying to get people to join a website how do I post a link to the website using my referral name or number? If anyone knows what I am asking, I could use some help.