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Create the user with is_active = False, Then you can easily filter those user on django admin panel.
Or you can use django signal such as post_save signal to send an email to administrator’s email, email can be included with an activation link which is unique for activating the user.
You can create a separate model to keep inactivated user and for each record generate an random-hashed key. You can use that random-hashed key (token) in activation link that you sending to administrator’s email.
There is already some application that let you to handle registration, for example django-registration. But your case it little bit tricky. Because you want to admin be able to activate those users. So I suggest you to look at django-registration source code. It’s so clear and easy to understand. Just read the code and you will get the point.