Q. How can I use PowerShell to get a list of all Exchange users who are hidden from the Global Address List (GAL)?

A. When a user is hidden from the GAL, the HiddenFromAddressListsEnabled attribute is set to true. Thus to find a list of all users hidden from the GAL, you just list all users with this attribute enabled, which you can do by using the following command:

Get-Mailbox | Where \{$_.HiddenFromAddressListsEnabled -eq $True\} | Select Name