I have wasted two whole weeks of my life on the infuriating

I have wasted two whole weeks of my life on the infuriating membership plugin. Nothing I do seems to work. Actually Nothing I do WORKS!!

I have set up a new site at caucus.eu/admin
I want to restrict access to my admin team only
Whatever I do I cannot get the registration page to work

I want you to logon to the site as me using the
[removed]

and find out what the problem is before I go out & shoot somebody!

Please treat this as an urgent request

I don't want to use coupons
I do want to use invite codes which expire once used
I don't want any visitors to see anything other than the registration page
Obviously if an intruder turns up they cannot register without an invite code
I want registered users to be able to see all content
I don't want anyone else to be a Wordpress Admin - not even the registered admin members
I just want to control the entire site myself but allow access to invited people of my choice (using invite codes)