No, it isn't.
Yes it most certainly is, though it is a very different kind of "registered member."
No, it most certainly is not. It's by special invitation only after passing a rigorous screening process. The fact that so few are invited should tell you something about the quality of the "registered member" pool.
Is there a list of "true believers" and just what is the tests?
There isn't really any formal testing process for becoming a true believer. However, you must convince the shadowy Zetetic Council™ that you sincerely believe that the earth is flat and can further advance the cause of FET. The vetting process isn't easy and few have what it takes to earth true believer status. Devil's advocates need not apply.
Is there really a shadow society?
Is there really a zetetic society?
Yes, and yes.
How does one contact the shadow society?
You don't. They contact you. If they think that you're worthy, that is.