There are 3 different types of groups that you can create in your community: Public, Private, and Secret.
1. Public groups are accessible for all community members, and anyone from the community can join the community anytime. Public groups get displayed in the group list in the "all groups" tab.
2. But, if a community members wants to join a private community, he/she sends a request to join and the group admin can either accept or reject the requests. Private groups contents are only available for their group members. Private groups get displayed in the group list in the "all groups" tab.
3. Secrete groups are like private groups, with this difference that non-members are not able to see a secret group in the list of the groups in the "all groups" tab. The only way to become a member of a secrete group is through the invitation that admin sends to the community members.