Let's say you have a Category called "Start Here"
And under that Category, you have a 'board' called "Welcome"
To set the board "Welcome" so visitor's can read/see anything...
Admin>Boards>Select 'modify' for the 'Welcome board'
Leave 'Permissions' alone.. instead, just below that is a series of check boxes.
Simply check and un-check who can see this board.
If you only want registered people to see a board, simply un-check 'Guest'
A 'Regular' member is one that has registered, all other member types are based on the number of posts.
You'll have to do that for every board you wish to hide/make private.
I'd suggest you keep at least one board visible, with a post about your content so people that come by have an idea what your doing.
Good luck!