Add field to customer sign up

ZC 1.3.0 installed.

At the customer log in step, I need to add Join and No Thanks buttons for a "Join Email Bargain Club" choice, plus a textbox for the new customer to indicate shoe size.

I'm trying to implement in the cart the feature from my client's static site at http://old-pine-footwear.com/bargainclub.htm The bargain shoes would not be from the on-line catalog. They are individual sizes/styles chosen from the brick and mortar store with a notice emailed to each customer with that particular size. I plan to make those shoes a separate category of products and hope to be able to restrict access to those pages to email bargain club members.

Forum search netted me a couple of somewhat related threads from two years ago and weren't really close to what I think I need.

Where are the files I need to modify, please.

Thank you,
Sue