Taken from: The WordPress MU Manual
Add New Users
Add new users controls whether blog owners are allowed to add new users to their blog. The default setting is Yes. Most circumstances you would allow blog owners to add new users.
Banned Names
You can ban specific names in WPMU so that no one can register with that username or blog details. By default, WPMU bans www web root admin main invite administrator. Examples of names you might add to banned names include ceo faq support.
Limited Email Registrations
Selecting the “Enabled. Blogs and user accounts can be created” option (Site Admin > Options – Allow new registrations) means anyone can create blogs using your signup page (e.g. http://writerspot.blogs.mu/wp-signup.php).
There are situations where you’ll want to restrict sign ups to specific email domains to limit who registers on your site. You do this by adding domains to the ‘limit email registrations’ section.
Banned Email Domains
Spammers can be a nuisance when you allow anyone can create usernames and blogs using your signup page (e.g. http://edutags.net/wp-signup.php). One way of reducing spammers is to ban email domains that are frequently used by spammers.
For example, banned emails domains on Edublogs include: wilemail.com, 21cn.com, pookmail.com, mailinator.com, disposableinbox.com, recclick.com.
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July 22, 2009 
James Farmer on Google+
T@wpmuorg Thanks for this useful info!