Is this at all possible to do?

[ninjashadow80]

OK

My site has decided to go private to where only members who are referred can join

When people join there is a spot to put who referred you. Is there a way to edit that to where it makes it required when registering? We wanna make it to where if you can't list a person who referred you you can't join

Can that be messed with?

Can the registration application be messed with at all

Please be as detailed as possible

Thank You