How to use Auto Action to restrict New User registrations (in Backend

Hi Kyle,
I wanted to restrict the registration of new users once the users are higher than a field which I set. How can I interrupt the creation of a new user using Auto action?
Or any other ideas how to restrict user registrations beyond a certain limit?
Or would it work if I all users who are beyond a value of a community builder field get promoted into a group which will effectively not allow them to register (ie outside of "registered" group)?

A user can't register twice. A bit confused on what you're trying to do. You're wanting to block backend user create for some admins?

I am basically "renting" out my website and the person who rents it has to pay by user. So if he pays to have a maximum of 20 users, then I had imagined that I set a CB Field to have a value of 20 (he cannot change that value because he has no access to it) and he cannot create anymore than 20 users.

Is that possible somehow? Can you think of how to make it possible?

Please note, I cannot simply bump him to a user group that is not allowed to create users in the backend because then he would loose a lot of privileges that are associated with the current user creation role.

Kind regards,
Tim

Yes that's possible. You'd need to act on the backend user create trigger before any user stores have been made. Specifically the onBeforeNewUser trigger. To block it you need to throw a plugin error.

Global
Triggers: onBeforeNewUser
Type: Code
User: Self
Access: Everybody
Action
Method: PHP
Code:
If you want this to also apply to frontend then add onBeforeUserRegistration to the triggers list. Note you will need to add your condition to check against the create limit or do so in the code it self. If the user limit field is on a specific user then you may want to set User to Specific and just specify that users ID to make substitutions be based off that user so setting up your condition would be easier.