Ploneboard 1.0's default workflows don't allow you to make an entire board members-only.  But it's easy to change this behavior.  Here's how.

1. Go to portal_workflows -> ploneboard_workflow -> states -> private -> permissions
2. Give the Member role Access contents information, Ploneboard: search and View permissions
3. Save your changes, then go back to portal_workflows and hit the Update Security Settings button at the bottom of the screen.
   

This will make dicussion boards that you put in the "private" workflow state visible to members only.  However, the forums within that discussion board may not yet be secured.  You will need to set any existing forums to "private" in order to prevent anonymous visitors from being able to deep-link to conversations and comments.

If you want to make the default workflow state for new forums to be private instead of memberposting, you need to change the initial state of the ploneboard_forum workflow.  To do this:

1. Go to portal_worfklows -> ploneboard_forum_workflow -> states
2. Change the initial state from memberposting to private.
3. Save your changes, then go back to portal_workflows and hit the Update Security Settings button at the bottom of the screen.