Can I do that with nodeBB?
You can create a private group, invite whoever you want to that group. Then in the ACP, create a category you want to be private and set the permissions so only people in that group can access and post to the category.
@pitaj great! I did not see this feature in discuz or wordpress.
@pitaj it looks not all people are allowed to do this in the forum.
No, this is a functionality only available to administrators.