Only another network administrator can create a network administrator. Go to:
Network Admin > Users
Locate and edit their user profile and tick the box allowing Super Admin privileges.
Important note: this should not be done lightly. Only grant super admin privileges if the person is going to be responsible for managing the network. An untrained super administrator can wreak havoc very easily on your network. Consider attendance on a Creative Blogs WordPress Multisites Admin course as mandatory before granting someone the ability to manage your entire WordPress Multisite platform.
When a network administrator leaves it is essential that you revoke their network admin privileges as well as changing the email address and password.
Network administrators MUST have strong passwords.