MyBB Community Forums

well i think you can do this in this way as i am doing...........

Create a new Special User group then create the forum you wanna hide from guests and other members....
just edit the permissions to newly created forums by allowing to "Special User Group" to see posts threads etc and disallow other user groups.

i want my forum to be visible only to registered users. How do i go about doing it?

ACP > Users & Groups > Groups > Guests > Forums and Posts > Can view board? > *untick*.