someone buys account through your download link, and you get 1/3rd of the value of that account.
Example: user bought 1 month RS account, you get 297 points, which is approx 1/3 of 990
(not really, they steal you 33 points from promised,but meh). You can have max of 99 'friends', also they count renewed accounts too. If user gives up, and doesn't buy account when it expires, he gets detached from you as a referral, and his place is free for someone else.

You can also transfer points to someone else's account, and make any free account premium.