There is this plugin: https://github.com/yariplus/nodebb-plugin-modmin
Basically does what you want, allowing you to give someone a permission to manage a specific category - including creating subcategories there.
so in nodebb terms, the category moderators and/or category admins can be given access control to manage the category
including who to allow/disallow into that category.
To elaborate on my last point, that would require giving someone global admin access in order to allow a group access to a category. You would then manually add members to that group that needed access to that category.
Yeah, this is tricky -- having individual moderators manage whitelists (either by user or by group) is definitely not a typical use case for forums, much less subcategories.
As @a_5mith said, it's possible to do if every global moderator has access to the groups page, and can manage their own groups, but at this point, there's a lot more work that needs to be done before we can even approach doing something like this...