This page uses the subpage function to pull signup fields from another page, in this case the join page.
Based on the Weblink:
http://nationbuilder.com/theme_documentation#display_a_form_from_one_page_on_another_page
Note that the landing page after you fill out the fields is the join page, so something to review at some point.
The fields below are pulled in via the subpage function in the template of this page: