Hi

I wan't add a refferal's box in "register.php" page like in this site:

http://gptreasure.com/register.php


Can any one help me ??

You would need to hack the registration form, and perhaps add some space for it in the db, to make a log the refer.

A possibly easier method would be looking into a profile module, something like that offered by Smart Factory, I'm not entirely sure as I haven't used this module, but I believe it allows custom fields, you'd have to try it and see though or ask on their support forums:

Smart Factory - Smart Profile

If your intent on hacking the form yourself, then there are many posts within these forums which would offer some great assitance.