membership

I have an interesting problem that one of you may have solved already. I run a merchandise site for a band. We are going to be offering a discount to members of their fan site (which I do not run). I plan to use the group discount option for this. The question becomes is there an easy way to update their customer profile to reflect they are members of the fan site. I can get a daily list of members of the fan site. I'm looking for a fairly automatic way to make this work because we currently have about 40,000 customers and the fan site expects to get 150,000 to 200,000 members in the next two months. Any suggestions would be great. we do not want to use a coupon code because it can be passed around and used by non members.