This FAQ is useful if you want to create a forum that only some users can access (its more easy than adding permissions per users manually).
First, the forum:
1- First, create your forum (Staff Forum).
3- Change the permissions of "Staff Forum" (Go to Admin CP => Forum Management => Permissions) to Private (or Private[Hidden] if you do not want other people to see the posts in this forum).
Next, the group:
1. Visit the ACP
2. Find the section 'Group Admin' on the left menu
3. Select 'Management' from the options
4. Select 'Create New Group'
5. Enter a group name along with a description, if required, and a group moderator and specify if the group is 'open' = anyone can join, closed = no one can join but they can request membership, hidden = it's not shown on the 'Usergroups' page unless your an admin
6. Click 'submit' to create the group
7. Click the link for 'Permissions' in the 'Group Admin' section
8. Select the group from the list
9- On the permissions page change the 'Simple Permissions' for the forums you want this group to enter from 'Disallowed Access' to 'Allowed Access'.
Done, now only members in that group can acces the Forum Staff.
Note: The group creation guide was thanks to UseLess.