single product signup

Hi,

I am looking for a way to modify the signup page so that it only displays a single product. I know that price groups can be used, but all my products MUST be in the same price group to ensure that the subscriptions work properly. So can I pass a parameter to signup.php so that it displays only a single product?

I'm happy to change the PHP code if required

Thanks in advance
Chris

Sorry Alex, I was getting mixed up with renewal groups I have put my products into appropriate price groups using multiple, comma separated numbers and can now display exactly what I require using price_group=XXX

thanks for the response