As admin, you can set user permissions in the top / outer level / root / whatever:
Options > Users > Change permissions > View... and select
who can view this application and its contents...
So ensure that
'Anyone' is not selected.
As registered users are all grouped as 'Registered', you may opt to select manually who is allowed access and put them into the Members group, or another group of your choice...
Options > Users > Extras & Add-ons > Email & Notifications and select...
Notify me when someone registers
When a user completes the registration process, an email is sent to all administrators with the details of the user (e.g., username, email, link to profile page).
Anne