Depending on your specific needs, you may find yourself wanting to change how users are added to sites, as subscribers, within your network. For example, by default, users cannot add themselves to a random network site without making the request to the Network Admin, or the site administrator of the site they want to be added to. This may work fine for most sites, but if you want your users to be able to register with existing sites within your network, then read on because this section is for you.
For many people, signing up to the main site and then needing to be added to a child site can be confusing. Plugins, however, can make the process easier and less confusing for visitors.
If you want existing users to have the ability to add themselves to existing sites on the network, the Add Users widget plugin, available in the WordPress Plugins Directory at http://wordpress.org/extend/plugins/add-users-sidebar-widget, will allow them to do so. Install this as a regular plugin, which we outline in Book VII, Chapters 1 and 2.
When the network is activated, the plugin adds a widget on the Widgets page of every users Dashboard (DashboardAppearanceWidgets) called the Add Users widget. Drag the widget to the appropriate sidebar to display it on the sidebar ...