User Configuration > Administrative Templates > Microsoft Outlook 2016 > Miscellaneous
User
HKCU\software\policies\microsoft\office\16.0\outlook\options\mail
This policy setting controls whether Outlook users can expand Contact Groups when addressing e-mail messages. If you enable this policy setting, Outlook users cannot expand Contact Groups. If you disable or do not configure this policy setting, when Outlook users add a Contact Group to the To, CC, or BCC fields of an e-mail message or other item, they can expand the Contact Group to see the e-mail addresses of everyone in the group.