When you enable the network, new site and new user registrations are turned off, by default. However, the Network Admin can add new sites and users from the Network Admin Dashboard. To let users sign up for your network, complete the following steps:
The Settings page loads in your browser window.
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.
By selecting the option: Both sites and user accounts can be registered on the Network Admin Settings page, you allow users not only to register a new account, but also give 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, ...