Until recently, you had two different kinds of groups and both could be managed through Exchange: Security Groups, and Distribution Groups. Now there is a new beast in the jungle called an Office 365 Group. An Office 365 Group cannot, as far as I can tell, be managed in Exchange. And in the general Office365 Admin Portal, there is no way provided to hide an Office 365 Group from the GAL. If anyone has a way to do this, either via the GUI or via PowerShell, I would love to know!