Thank you all for your comments.
Based on your feedback, we are updating the default privacy setting for groups created across all five Outlook endpoints (web, PC, iOS, Android, and Mac) to be private by default where only approved members can see what’s inside the group. For customers that want to continue with the existing behavior of public by default (where anyone in your organization can see what’s inside) we are providing an Exchange Online PowerShell cmdlet to define the default value. This update will gradually rollout to all Outlook endpoints in the coming months starting with Outlook on the web.
Keep your feedback coming.
This change is now rolling out, please see this post for more information: https://techcommunity.microsoft.com/t5/Office-365-Groups/Groups-in-Outlook-and-Group-connected-team-sites-are-now-private/m-p/186395#M5266
Thanks for all the feedback on this item. We’ve started initial thinking and investigations on possible designs for this feature.