Actually, you don't. 

You get ScrapPoints when your friend registers to become a member of the Club Scrap^® Community and ONLY if they do so by registering through the link in your special invitation email.

You will not get points if they come to the site and register directly. They must visit the site through the link in the invitation. We can not go back and add points if that is not made clear to your friend.

There currently is no limit to the number of times you can use this source of ScrapPoints and the Friend Request is customizable so you can be very clear in explaining the process to your friends.