[spooler]

Just my opinion here..

A url like this

register.php?invite=yes?referrerid=$bbuserinfo[userid]

can easily be made by any 1...

register.php?invite=yes?referrerid=1

The above url will work and will most prob be invited by admin himself (or who so ever has id =1)

So basically any random no. between 1 and total members of board will work.

Just my opinion :

So basically u need to have another table named invitee with fields ( random number(unique), timestamp)

When u generate invite url, add new row in the table and your invite url will be register.php?invite=random number

In register.php make a query to see if tht random number exists. If it does allow else kick him away

Do remeber to clear the row once used.
Also add to cron to clear rows in invite table with timestamp older than 1 day or whtever