The way you set the vCenter Server administrator user depends on your vCenter Single Sign-On deployment.
In vSphere versions before vSphere 5.1, vCenter Server administrators are the users that belong to the local operating system administrators group.
In vSphere 5.1.x, 5.5, and 6.0, when you install vCenter Server, you must provide the default (initial) vCenter Server administrator user or group. For deployments where vCenter Server and vCenter Single Sign-On are on the same virtual machine or physical server, you can designate the local operating system group Administrators as vCenter Server administrative users. This option is the default. This behavior is unchanged from vCenter Server 5.0.
For larger installations, where vCenter Single Sign-On is part of the Platform Services Controller and vCenter Server are deployed on different virtual machines or physical servers, you cannot preserve the same behavior as in vCenter Server 5.0. Instead, assign the vCenter Server administrator role to a user or group from an identity source that is registered in the vCenter Single Sign-On server: Active Directory, OpenLDAP, or the system identity source.