The administrator may have enabled referrals, which allows you to be credited for any new users you invite to the forum.
You gain referrals by promoting $vboptions[bbtitle] using your unique referral link. The link you should use as your referral link is this:
$vboptions[bburl]/$vboptions[forumhome].php?referrerid=$bbuserinfo[userid]
Each time someone follows this link to $vboptions[bbtitle] 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.
Your administrator will have more information on any prizes or rewards that may be given out for referrals. Please be courteous when promoting $vboptions[bbtitle] by not spamming other web sites with your referral link just to gain referrals.