User Registration Invite Codes add-on Nulled allows you to restrict the registration on your site to users with invite Codes only. You can create a single or multiple invitation code, set expiry date, usage limit per invitation code and more. Also have the option to generate random invitation codes in bulk amount. No hassle to create one at a time if you have multiple codes to be created. Easily add the invitation field in your registration form by just dragging the field in your form and that’s it. Only users with invitation codes are allowed to register and others will be denied for registrations.
Demo : https://wpeverest.com/wordpress-plugins/user-registration/invite-codes/