You can tell them to enter on the sign up page there is a field that says ' refer by'

Ask them to enter your nickname there.

OR

You gain referrals by promoting VIPrapid.com using your unique referral link. The link you should use as your referral link is this:
http://www.viprapid.com/index.php?referrerid=1
Each time someone follows this link to VIPrapid.com and registers as a new member, your referral count will be incremented by one. The number of referrals you have can be viewed in your profile.