Groups which are not ready to be announced, or need to stay hidden for any reason, can be set to private from the group settings.
Some types of leadership groups or recovery groups can be taken off the group directory but still accessible to members of the group. In addition, private groups are good to give administrators time to set up groups before they're announced to the public – a way to "stage" a bunch of groups and then launch them all at once to the public.
Groups can be made public or private on the individual group page from the settings.
They can also be made public or private all at once.
Private groups are only known to Administrators, the group's leader(s), and the members of the group. The URL of a group page will redirect someone not logged in to the main group directory. However, if members (including leaders) of that group log in, they'll then be able to get the URL of the group page directly.
On the group directory, members of the private group can see their group in the "My Groups" section, but non-members will not be able to find the group even in a search.
If all the groups in a given group type are private, the group type itself will be hidden from the group directory.