How to Allow User Registration on Your WordPress Network
When you enable your WordPress network, new site and new user registrations are turned off, by default. However, as the network administrator you can add new sites and users from the WordPress Network Admin Dashboard. To let users sign up for your network, complete the following steps:
Click the Network Admin link at the top of the Dashboard and then click the Settings menu link.
The Settings page loads in your browser window.
In the Registration Settings section, select the User Accounts May Be Registered option.
This allows users to register on your network, assigns them to the main site as a Subscriber, but doesn’t allow them to create a new site of their own.
Click the Save Changes button at the bottom of the page.
Both sites and user accounts can be registered on the Network Admin Settings page; you have allowed users to register a new account and given them the option to create a new site on your network.
When signing up, users are directed to the main site of the installation and then added to one of the child sites. This site may be their site (if they choose to have a site when registering) or an existing site. If it’s any existing site other than the main site, the user must be manually added to that site by the network admin, or the user who owns the site (if you have enabled the option under the Network Admin Settings to allow site admins to add new users to their sites).
After filling out the form, the user will receive an e-mail with a link to activate her account. When she does so, she can immediately log in and manage her details; she is directed to her primary site, which is the main site or Dashboard site if she has no site to administer.
Users can also be added to existing sites in the network. The network admin can always assign users to specific sites on a per-case basis.