You would need Data Bridge as well in order to send the emails.
You could create a separate admin login that allows administrators to login and approve user registrations.
It would involve probably adding two fields to the users table "UserApproved" and "UserIsAdmin". Then you could default to zero and filter in the user login based on the UserApproved column value being "1"... Admin login could do the same with the UserIsAdmin column.
Then the admin login could take the users to a list of the users identifying the ones that need approval based on the UserApproved column and have an update page where they could set the value to "1" based on a checkbox.
Emails can be sent along the way by adding the "Create Email Message" server behavior wherever appropriate (on registration and admin approval pages).