Private Boards are visible only to users added by Board Owner to it. They are not visible or accessible to users which are not added. You must be added by Board Owner to become a member or visitor in this Board.
1. Adding a Private Board
Private boards are added as any other board.
All boards are created as private bz default.
Simply click + (Add Board Button) in any Board Group.
Provide information required by the form and create.
A newly created Board won't be indicated as a Public one.
If a Board is private, it won't be indicated on your KanBo Home (while you look on the board tiles). Only public boards are indicated by special information on tiles.
2. Adding users to Board as Owners, Members and Visitors.
The Board won't be visible on the Personal KanBo Home until you got added to it as Owner, Member and Visitor by a Board Owner and add the Board itself to the Personal KanBo Home.
Here you can learn how to add users to Board.
Change Board's type
If you wish to change a Private board to become a public board, you can enter Board Settings -> General and change that simply in the Visibility section.