ESXi handles user authentication and supports user permissions.

ESXi 5.1 is not integrated with vCenter Single Sign On and you cannot create ESXi users with the vSphere Web Client. You must create and manage ESXi users with the vSphere Client. vCenter Server is not aware of users that are local to ESXi, and ESXi is not aware of vCenter Server users. However, you can configure Single Sign On to use an Active Directory domain as an identity source, and configure ESXi to point to the same Active Directory domain to obtain user and group information. This action allows the same set of users to be available to the host and to vCenter Server. For information about vCenter Single Sign On, see Configuring vCenter Single Sign On.