There are a few ways that you can limit signups to your WordPress MU site – via email address or limiting signups to existing members (or just yourself) only.
This plugin adds another way – the ability for you to set a signup code that stops any new visitors from creating a blog unless they know the code!
Introducing the WordPress MU Signup code plugin!
It’s effortless, just drop the plugin into mu-plugins and enter the code you’d like visitors to provide before they can create a blog or username and edit the text you want to appear above it (the ‘branding’).
After this, your sign up field will now be modified (remember to log out to see it in all its glory), and if visitors don’t enter the correct code then no blogs for them!