When in the admin panel - within the 'Configuration and Settings' page you have the option of enabling the Organisation Approval.
If 'YES' is selected, users can register for your platform but will then be taken to a holding page whilst they await for approval.
An admin can approve them on the admin panel home page.
If 'Nobody' is selected, it will simply turn off the ability for new users to register, and when clicking the registration link they will be taken to an error page.