As the Office admin GUI does not offer the option to hide email addresses from the Global Address List (GAL) administrators must use powershell to complete the task, as is the case with many other features and functions in O365…
Fire up an elevated PowerShell window.
Set the execution policy if required, answer “Y” for Yes or “A” for All.
Set-ExecutionPolicy RemoteSigned
Create and Import the session and login for the O365 instance.
$session = New-PSSession -ConfigurationName Microsoft.Exchange -ConnectionUri https://outlook.office365.com/powershell-liveid/ -Credential $Credential -Authentication Basic -AllowRedirection Import-PSSession $session
Set the -HiddenFromAddressListEnabled option to True – ensure to change **EMAIL-OF-GROUP** to the correct emial address
Set-UnifiedGroup -Identity **EMAIL-OF-GROUP** -HiddenFromAddressListsEnabled $true
Remove the session.
Remove-Session $session
And thats it. You’ve just hidden a group email from the GAL. Remember though, users will alos use the Offline Address List which dependant on your policies, will take a short time to update.