I have a private forum I use for prepping posts for my (currently on hold) Bible study threads and other posts that require some thought and effort before I want them to be visible. It is set to private with only me having rights. I type and edit the posts there, then move them to our Religion & Faith forum when they are ready to go. I also installed a free date editor app so I can change the date when I move it since I sometimes prep it mid-week but don't release it until the next Sunday.
Basically, in Node Permissions for the node, I checked "Private Node" (or whatever it says, can't get into my ACP to check right now) then added myself as a user with rights to post, edit, etc. You could also create a group and assign the rights to that group if it was several people. Then if someone needs to be able to work in that private forum, you just add that group to their account.