I’ve been very fond of a plugin named new user approve written by Picklewagon. It allows you to open registration, and over-rides the default registration process of automatically allowing the user in and sending you ( the admin ) an e-mail — along with the newly registered user.
I’ve also been digging deep into WordPress multisite / multiuser, and have found that this plugin is not compatible — and no other plugin is, for this type of situation! Going through the forums, I found that there’s a great need for this in WordPress MultiUser/MultiSite. For that reason, I decided to go ahead and research what was causing the lack of execution of the approval process in registration. Come to find out, WordPress’ register_post and registration_errors are not MultiSite / MultiUser friendly! Even WordPress 3.3.1 is still tied to the wpmu_validate_user_signup hook.
Luckily, the fix wasn’t too bad — I took the code for register_post and registration_errors and put them together in a new function call which I hooked to wpmu_validate_user_signup.
That did the trick! This was a cool learning experience for me as I hadn’t dabbled into this type of functionality before and I have to say.. WordPress, I <3 you :).
P.S. The goodies — I have zipped the plugin directory for your using pleasure, here: http://beshoy.girgis.us/files/2012/02/new-user-approve.zip