The difference between public, private and secret groups

A group has 3 levels of confidentiality, selected when the group is created.

The 3 levels are as follows: 

• Public: the group is visible to all. Content is only visible when you join the group, by simple membership, without validation by the group administrator. 
• Private: the group is also visible to all. The list of members and content are hidden, and access is only possible by requesting to join the group.
• Secret: the group is invisible and not visible to everyone on the site. Access is by invitation only.