There are 3 privacy setting options for a group. They differ with how searchable or visible they are in the app and to non-members and with how users join. 

Public: Any user can find, view and join. The group's content is visible even if the user is not a member of the group.

Private: Hides the group from the internet. An app user can search and find the group. The group's Public Dashboard will display to an app user who is not a member of the group.

 The Approval Required setting is "on" (enabled) when a new subgroup is created with Private visibility. The group admin is responsible for accepting join requests. We recommend that the Approval Required setting is "on".  

 If a group admin changes the Approval Required setting to "off" (disabled), then an app user who finds the private group is able to join automatically.

Secret: Nothing about the group is visible in the app and on the internet. A member is invited to join.