Tracking sales to a group of customers

Hi,
I am trying to set up a cart for a fundraising company. I would like to offer customers that purchase my products for fundraising the option to let people come to the cart, make a purchase, and their group would get credit. I can give the group a code to use when they get to the site, but I don't see any way to have them enter it. I was thinking that I could force an account to be set up (no guest purchases), but I would really need another field for them to enter into the user account that would then carry through with the data that I get when the order is placed. Also, I think this would also work if I could set up a drop down option on the checkout, but I haven't seen that option either. Any ideas or examples on how to handle something like this? I am running 1.4.9.1.
Thanks,