A user is an individual authorized to log in to ESXi or vCenter Server.

In vSphere 5.1, ESXi user management has the following caveats.

You cannot create ESXi users with the vSphere Web Client. You must log directly into the host with the vSphere Client to create ESXi users.

ESXi 5.1 does not support local groups. However, Active Directory groups are supported.

To prevent anonymous users such as root from accessing the host with the Direct Console User Interface (DCUI) or ESXi Shell, remove the user's administrator privileges on the root folder of the host. This applies to both local users and Active Directory users and groups.