When you first create an Office 365 group in Outlook, is it Private by default. This means only approved members in your organization can see what's inside the group. Anyone else in your organization who is not approved cannot see what's in the group. However, you can choose to make the group Public so others in your organization can see inside it.
For example, you created a group for the human resources team at your company and made it a public group, but now, you want to make it a private group. You can quickly and easily change the privacy settings in a few steps.
Check out Learn more about Office 365 Groups for details about Office 365 groups in Outlook.
Make a group private or public in Outlook on the web
Open Outlook on the web.
Navigate to the Office 365 group that you want to change the privacy settings on.
From the group page, click or tap Edit group.
Under Privacy, choose Public or Private to match the setting you want.
Click Save to make your selection, and close the page. It may take a few minutes for the changes to take effect.