Office 365 Exchange introduced a feature called Office Groups last year. Users can create private groups, accessible only to subscribers, however, these groups will be shown in the All Groups list in the GAL. The groups can be hidden in the GAL by setting the HiddenFromAddressListsEnabled parameter to true. (Set it to false to again show the group in the Global Address List.)
Set-UnifiedGroup groupname -HiddenFromAddressListsEnabled $true
It can take up to 24 hours for the offline GAL to update with the change.