If you refer Grabien to a new user, tell that user to enter your referral code during the sign-up process. When that new user creates his account, yours will immediately be given 100 bonus promo coins.
Even better: For every deposit this new user makes, your account will be matched 1-to-1 with promo coins. (As an example, if the user you encouraged to sign up buys 1,000 coins, your account will receive 1,000 promo coins.) Note that promo coins work just like regular coins, except with with certain features — such as Requests — the price is slightly higher.
Where is your referral code? At the top of your profile page as soon as you log in.