There are a few ways that you can limit signups to your Multisite or BuddPress network – via restricted email domains 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 an account 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 on Multisite and BuddyPress networks.
Once activated go to Network Admin > Settings > Sign up and add your code.
Now, 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 accounts for them!