Restrict MemberPress membership registration page

I’m using the excellent MemberPress plugin, but have come across an issue I’m struggling to resolve. Payment for my protected membership will be done elsewhere external to the site, but I need a way to ‘protect’ the MemberPress registration page itself, otherwise anyone with the registration url can sign as a new user and then have access to the paid content. Is there any way I could, for example, add an activation code or something like that, so that each genuine user that I wanted to be able to sign up would need an unique activation code in order to access the sign up on the registration page?

Any help or advice would be very gratefully received!