Private groups vs public groups
When creating a group you’ll need to decide if you want it to be a private group or a public group. Content in a public group can be seen by anybody in your organization, and anybody in your organization is able to join the group. Content in a private group can only be seen by the members of the group and people who want to join a private group have to be approved by a group owner.
Neither public groups nor private groups can be seen or accessed by people outside of your organization unless those people have been specifically invited as guests.
To change your group from public to private (or vice-versa) go to the group conversation location, either in Outlook or Yammer. For Office 365 Groups in Outlook, go to Outlook 2016 and choose Edit Group from the ribbon. On the group settings pane that appears you’ll be able to toggle the privacy setting. In Outlook on the web click the More group actions button at the top right (looks like three dots) and choose Edit group from the drop-down menu. For Office 365 Connected Yammer Groups, go to the group in Yammer and click the settings icon in the group header to edit the group’s privacy setting.