join with code instead of payment

I have a site where I would like them to Join using a code. I have it setup right now with a price, and a coupon, but it would be much preferable to be able to just put a code in, without a price, allowing membership. They codes do need to be used only once, which is why I was using the coupon code, but if I put price as $0, then the coupon doesn't show. I don't want confusion by the individuals signing up when they see a price. Ideas?

In this case, I some customization to the code will be needed. You can still create these codes using coupon but we need to modify code so that:

- The coupon input is always shown
- Users need to enter a valid coupon code to complete the registration (it's nor optional like standard coupon)

This custom development will cost you 40$. Let us know if you are interested

Tuan