Hi
The tabulation, there is no SSO on ESXi but you can use Active Directory.
As described in vSphere documentation here: vSphere 5.5 Documentation Center - Authenticating to the vCenter Server Environment
"In vCenter Server versions 5.1 and later, users authenticate through vCenter Single Sign-On.
The user administrator@vsphere.local has vCenter Single Sign-On administrator privileges by default.
When logged in to the vCenter Single Sign-On server from the vSphere Web Client, the administrator@vsphere.local user can assign vCenter Single Sign-On administrator privileges to other users.
These users might be different from the users that administer vCenter Server."
Basically after you install a vCenter, only the SSO Administrator which is administrator@system-domain (v5.1) or administrator@vsphere.local (v5.5) that has admin privileges to access vCenter Server.
If you want to assign SSO Admin or assign access to vCenter, you can assign user from SSO (domain vsphere.local/system-domain) or Local OS (windows OS or linux for VCSA).
You can also assign user from Active Directory or LDAP, but first you would need to add them to the Identity Sources first.
After added, then you can assign users from those Identity Sources as SSO Admin or to specific objects in vCenter objects.