Rad is a site that sells clothing for men and women. If you refer your friends you can earn free store credit.

Sign up then invite your friends to get your free store credit to Rad.co. You will get $30 if you invite 10 people, or $50 if you invite 25 friends.

It doesn’t state what happens if you get more than that, but I sent an email request asking. They wrote back that $50 is the maximum you can get.