Signup Code
Limit who can sign up for a blog or user account at your site by requiring a special code that you can easily configure yourself
Now compatible with BuddyPress too!
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.
T
his 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!
Alternatively if you are looking to control Splogs -- check out our Anti-splog plugin which is the ultimate plugin and service to stop and kill splogs in WordPress MU and BuddyPress
It's effortless.
Simple as drop the plugin into mu-plugins and enter your sign up code in Site Admin > Options

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!

Enjoy :)
If you like this plugin, you might also be interested in:
1. Set password on WordPress MU blog creation
- Allows users to choose a password at signup.
- Places a Terms of Service box on the signup form forcing the user to tick the associated checkbox in order to continue
- Allows new users to select the language they use at signup
4. Remove Email Verification from Signup
- Remove the need for email verification at sign up - now your users can set about blogging from the get go
5. Set blog description on blog creation
- Allows new users to set their tagline when they create a blog
