Need way to lock down signup for member for other site.

[Hugh Hou]

Is there possible to lock down registration with a special code? I just purchase this plug-in. And I need to build an membership site that only allow members from other membership site to signup. The signup is not open to public. So when user click signup button, I want to ask them to input a special code before they can signup. Or if they are redirect from a certain URL – so I can create a button with this special URL to embed on the other membership site member portal. Can this plug-in do that. If not, what is the solution? Thanks!