The initial user account that is created when Mura is first installed automatically receives the special designation of "Super Admin." In addition, any user that you create can be designated as a Super Admin by going to the Advanced tab of the System User Maintenance Form. These users may, but do not necessarily have to, belong to any other group(s).
Users designated as Super Admin have the highest authority, rights, and control in Mura CMS. Super Admins can add/edit sites, edit Site Settings, create/manage System Groups, Member Groups, add/edit users, has full access to the File Manager, can install/delete plugins, and much more.
Yes, Super Admins have great power, and as the old saying goes, "With great power, comes great responsibility."
Obviously, anyone you choose to designate as a Super Admin must be someone you can trust.