Unauthorized registration

I am attempting to set up a website that is free, using the “Membership” plugin, but only for people that I give an Invite Code to. That part works fine when they use the link on the front page menu, but while testing it I found that someone can register without an Invite Code by using the register link under the WordPress Login Box because it takes them to a Register page without the Invite Code field. How can I prevent this unauthorized registration?