WordPress Multisite has an additional admin role above regular admin, called the Super Admin. Regular admin user can manage and administrate a single WordPress site and the Super Admin user has admin rights to all sites under the multisite installation as well as to the network itself.
You can create a new Super Admin user in your WordPress Multisite by navigating to My Sites > Network Admin > Users -tab and selecting the Add user button. After adding the new user you can change their user role from the Edit user -menu.
Adding a New Super Admin to the Network
1. Start by navigating to the Users -tab.
2. Click on the Add New -button to add a new user.
3. Enter the username and email for the new user. After the user is added an email will be sent to them with a link for setting up a password.
4. After the user is added, click on the Edit user link in the view that opened up.
5. From the menu that opens up, scroll down to the Super Admin option, and select the Grant this user super admin privileges for the Network -checkbox. Finally, click on the Update user -button at the bottom of the menu.
Now the new Super Admin user has been added to the Network and they can start managing all the sites under the Network.
WP-CLI
You can also add Super Admin rights to existing user with WP-CLI by running command wp super-admin add <username> on the server.