You can store his details on firebase database, and then create another app for the admin to manually approve the accounts where in he will be able to see the details of the people and then approve that particular person or disapprove.
Try and create two project buckets with one containing all the unapproved users and the other approved. So the admin can see which users are unapproved and then after approval delete them from the unapproved bucket and add them to the approved bucket.
There’s a difference,
You see when you have apps for like a company you would only want the employees to use the app and therefore approving is must as any other person can use the app.
Also another example, I believe you must be familiar with Allen Career Institute. They give you the username and password for their online course after being approved so they can keep freebies away.