Issue: I want to have a forum that is only available to selected members.
You can do it by creating a new member group and moving them into it....OR NOT. :)

The first way to do it is to create a new member group for the selected people you want to see the special forum, and then move them into that group.

AdminCP > Users & Groups > Manage User Groups > Create new group (button at bottom) Set up the permissions as you like.

However, if you do not want your special members to look like they are in a different group, you can do it this way once you have created the new group:
AdminCP > Users & Groups >Edit/Suspend User > Find user > edit Details>

Check the box that says "Override Group Permission Mask with..." Give them the permission mask of the new group in the Permission mask override box by selecting the new member group. They will remain in the same member group they are now in, but have the additional permissions of the new group.

Then create the special forum:

AdminCP > Forum Control > New Forum
Set up the permissions for the special member group

Set up the permission masks for the special forums so that the new member group you have created can see/post/reply there. (Don't forget to include the Admin group)

AdminCP > Forum Control > Permission masks