This page describes the templates involved in generating the new account form, which is seen when clicking the "Get an Account" link in the "Account Options" nav box. The same templates also control the form seen when modifying your account.
add_user_header.php
This controls the appearance of the top of the form, above the first field.
add_user_fields.php
This controls the appearance of each field on the form.
add_user_footer.php
This controls the appearance of the bottom of the form, below the last field.