5
That's easy man.. if i understand you correctly
You create a special group of people in the groups admin of the system module.
Create a new group with the same permissions as registered members. Then in the forum admin, edit the forum and give only that new group the permission to post reply's and open threads.
Everyone that you want to be able to post in that particular forum you just add to that new group and he will automatically be able to post at that forum while everybody else wont.
If you use NEWBB 2 RC1 it is really easy.
I hope you understand me.
So, create a new group and assign only to that group the permissions to open threads and post in newbb2 while editing that particula forum.
Daniel