Lets say you have a membership level called “Invitation Only”. You only want users with a special password or code to be able to signup to this membership level.
Do the following to achieve this type of setup
Step 1) Create Your Special Registration Page
Create a new WordPress page and follow this instruction to add a registration form on this page (for the membership level you want).
Step 2) Set Your Special Code or Password
Edit the visibility of the page and select the “Password protected” option. Set your special code (invitation code) as the password of this page.
Step 3) Send Users to This Page
Now, you can communicate to your users and tell them to go to this page and use the code/password to unlock the page and register.
Only users with the correct code will be able to see the registration form and register.