To limit who can see a forum, you need either the moderator or operator role.

If your campaign has multiple storytellers, it might be useful to have a DM-only forum. You can set this up by setting the view rights on a forum.

You can set the view right by visiting to the Forum Management page and expanding the forum with the edit icon on the right side, or by editing the specific forum.

Once you have found the editor for your forum, you can select which roles are allowed to view the forum the forum, or even allow specific players to see it regardless of their role. If the list of allowed roles is not visible, the forum is visible by everyone. You can restrict that by first toggling the view rights, which causes the list of roles to be shown.

Note: Note: at this time, anyone able to see the forum is also able to post there as long as they are a player in the campaign and have not been banned.