This is the "join.atxhs.org" page. It presents a form to collect email address and other info, and triggers Freshbooks to send you an email with an invoice link.
It used to also create an entry in the Hacker Directory / LDAP system, but when that went offline, the code was editted by CoffeeIV to have an option for skipping that connection.
See https://github.com/pib/pennyworth-web and forks of that.
The code resides on the same server as atxhs.org.