Multisite User Registration Manager is a WordPress plugin by zaantar that provides a system for registration requests and their processing in multisite.
Blog administrator can place a shortcode [murm-form] on their blog, which will display a registration form. Visitors can then use this form to request registration. Such a request must be first moderated by the blog admin. If it is accepted, it’s passed on to the blog superadmin, who also has to approve it in order to pass. After approval the plugin creates given user accounts with subscriber rigths on the blog from which it was requested and sends login information to the user.
Makes use of the WordPress Logging Service plugin: http://wordpress.org/extend/plugins/wordpress-logging-service/