How to Turn Registration On and Off for WordPress Multisite
Long time users of WPMU may already be familiar with registration options, but there many new users to Multisite due to the merge with WordPress 3.0. Questions about how to turn registration on or off will occasionally pop up in the forums. The super admin can enable and disable registration for the site at any time. Go to Super Admin >> Options, as shown in the thumbnail to the left.
There you can adjust the registration settings to suit your site’s current needs.