Here's what you do:

-Go the main admin page.
-Forum Admin > Management > Create new forum (just like normal) > Submit
-Forum Admin > Permissions > Choose the new forum > advanced> make everything "PRIVATE" > Submit
-Group Admin > Management > Create new group (this is pretty straight forward. make the group hidden). Submit
-Group Admin > Permissions > select the group > give the group access to the folder

To add users, go out of the admin page, and click on "Usergroups" at the top of your forum. You should be a member of the group since you're an admin, so click on "View Information" for that group. At the bottom of that page there should be a text box for you to add usernames.

I believe that's all there is to it.