There are two main types of Community Groups, public and private.
All community users can join a public group by selecting the “Join group” button on the Group Page or via the invite they received.
A private group can be joined by invitation received in an email, a private message or by sending a request to join (selecting the “Send request to join” button) on the Group Page. While the group icons are visible to all members, its content can be accessed by group members only.