Networks
Network pages are simply a grouping of sites. The network approves sites' membership, but does not approve content created on the individual sites.
In the left nav, click the "Networks" tab and click "Add new."
If you don't see the networks tab, you need to activate the Network Landing Pages plugin, under "Appearance > Plugins". You have to be in the main site admin, not the network admin.
In the left nav, click the "Networks" tab and select the network to which you'd like to add a site. In the Sites section, check the box next to the site to add. Then, click Update.
Technically, a site can belong to more than one network. Whether they should is an administrative decision.
Disabling the Network Landing Pages plugin on the main site will remove the Networks section on the sites directory page.