Site Members / Shop Customers

I have been developing a site with a members section and shop. I am now at the checkout phase with the shop and am wondering what the best way to get customers to sign up with their address details for the shop part of the site.

I am guessing that if a customer is already signed up as a member to view content in a members only part of the site that using the same email address to signup to purchase items through the shop would throw up an error. To sign up for the members only area they dont need to enter their billing address.

What would be the best way to capture their address when they are going to order from the shop if they have already registered for the members area?