Office 365 – Create a Secret Group

Keep in mind that hidden membership can only be applied when creating a NEW Office 365 group, you cannot change existing groups.

# Create a New Hidden Group Membership
New-UnifiedGroup -DisplayName "Secret Group" -Alias "O365Group-Secret" -EmailAddresses "SecretGroup1@thecodeasylum.com" -AccessType Private -HiddenGroupMembershipEnabled
# Hide the group from the Global Address List and Search 
Set-UnifiedGroup -Identity "O365Group-Secret" -HiddenFromAddressListsEnabled $true

You can hide existing private Office 365 groups from search with:

Get-UnifiedGroup | Where {$_.AccessType -eq "Private"} | Set-UnifiedGroup -HiddenFromAddressListsEnabled $true