As of 23. of march 2018 all private teams will be searchable for all users.
Users can then apply for group membership.
If you have groups in your organization that you still want to keep completly private then the Office 365 unified group should be hidden from the Global Address List (GAL).
Run this Powershell command to hide it from GAL:
“Set-UnifiedGroup -Id “Groupname”-HiddenFromAddressListsEnabled $True”