Hi coolbreeze,
If you visit your profile (click MY PROFILE) and scroll halfway down the page you'll see your scores, your replay section and your friend section. Right below the pictures of your WGT friends is a button to invite friends to the game.
Just click that button and enter your friends email addresses. Each of them will receive an email invitation to join the site and the email will also include a referral link for them to click on to register for WGT. As long as they register using the referral link in that email you will receive a free 3-pack of GI-D balls and they will receive a free driver upgrade. If your friend goes on to make a $10 deposit you will receive a 200 credit bonus in your account.
They must use the link in the email to sign up otherwise you'll both miss out on the bonuses! Good Luck =)