GST-free customers

Many of our customers are entitled to make their purchases GST-free.

How can we get them to indicate this when registering, so that it will be applied at the checkout?

Any help much appreciated.

I know that to do this, I need to add a Y/N select box to the registration page that will enter the choice into the tax_exempt field of the users database table.

I just cannot identify the file which I need to modify to do this.

Can someone please help?