How to add a new super-admin to a WordPress multisite
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 menu 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 menu.
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.
You can also add super-admin rights to existing user with WP-CLI by running command
wp super-admin add <username> on the server.