I need to extend the creation of user accounts in a bit more complex way:

A new account must be approved in two steps. First, one of a number of different group-administrator (lets say groups A, B, C, D) has to approve, and second a central administrator has to finally approve the user, too. So, when a new user tries to create a new account, he has to choose his prefered group (from the above e.g. 'B'). Then a "please approve"-email goes to the relevant group-admin (group-admin of group 'B'). This admin has to approve or deny the new user (mail is created to the user, if group-admin's decides to deny). If the group-admin has approved, a "please approve"-email is sent to the central-admin who has to finally approve the new user (also a welcome or deny-email is sent to the user).

Group admins and central admin should not be site admins. They only should have the rights to approve or deny the user-accounts. Group admin and central admin should have access to a special list of members that they have already approved or which are waiting for approval.

Do you think this can be done with existing Drupal modules, or is it nessecary to code a new module? Or at least: are there any modules, on which this user accounts can be build?

Thanks in advance for any help.
Best Regards
Armin