If user2 goes to your site because of a link on user1's web site and leaves you have no information on him. How are you going to know when he comes back? Do you have a login page? Is there anyway you can store his info if he(user2) does not fill out the form.
If you log everyone that comes to your site from user1 that file will get large and how do you know when to delete them?
Say user2 comes in from user1 and leaves; comes back 5 times over a three month period and then fills out the form. See what I mean?
I think you can reward user1 when a person follows his link to your page and you can give out bigger rewards if user2 fills in the form at that time, but keeping records on people that enter via a link and tying them to that link will get out of hand.
Session variables are destroyed when a session ends or times out.
Before starting an online advertising network learn how to use a database!!