Signup when using iThemes Security redirect?

I have a multisite setup using iThemes Security, which uses a different URL than the standard wp-admin and wp-login.

I'm trying to set up a Jobs & Experts instance on one of the multisite instances, but new signups are getting redirected to the standard wp-signup.php for the overall site. Which of course gives them a 404, since it's protected by iThemes.

How can I allow them to sign up as a user just on this site?