This is a how-to question, unless it doesn’t exist in which case it’s a wish list item. 
I would like to have the same document/file be included in multiple folders/outline items.
I want multiple references to the same file as opposed to separate copies. I.E. when I edit
one, I see the changes in every folder in which it is included.
So, how can I “share” a document/file between multiple folders?
The current answer for this one is that you can’t.
It may be possible for you to setup some saved searches that work the way you want (these act somewhat like smart folders).
I would also find this a very useful feature. Is there some way to put it on a request queue for future enhancements? I have no idea how Scrivener implements and maintains the file/folder structural relationship: implementing this kind of “symbolic association” could be trivial or very tricky, but if it’s a relatively simple enhancement, I’d like to suggest it or vote for it. 8/
You can achieve something like this at the moment with collections, though without levels. Put all the documents (of any folder in the binder) that you want in your outline into a single collection. They remain in their actual location in the binder, but you can see them grouped however you like this way, and changes made will naturally affect the document wherever you’re looking at it. Although you don’t get the subdocument structure of the binder, depending how you work visually you may find that using the freeform corkboard for this kind of structuring will work, and that is available in a collection (click on the title bar of the collection when it’s open in the binder).
I realize it’s not precisely what you’re after, but try it out; you may find after you’ve played with it a bit that it will achieve what you need. (And it’s available now, unlike the wished-for capability! 
)
There is a wish list forum for that kind of stuff. Keep in mind the notion of symbolic links has been brought up before several times on the forum. Ultimately, the answer to that was collections. The main problem with implementing symbolic links as in other programs that feature outliner clones is that the binder was never coded to support that at all, and at this point in time it would take a total rewrite of the entire thing to accommodate them.
Do play with collections though. I’ve found that in most of my own cases, in the types of places where I would want to see resources twice, that whole desire can be addressed with a list that I can curate. It’s a slightly different way of thinking about things, but it can work, and the benefits of using these lists are many. They are integrated very well with the whole application.
The other problem with symlinks is more practical. Introducing a feature like this would increase support burden as any sane implementation of it would (a) be easy to accidentally do, and (b) not be immediately obvious that the item you are looking at is a clone (things like this need to be really obvious to avoid becoming a common FAQ level thing). Collections are way safer in this regard, because creating them will never cause a “Why is chapter 20 compiling six times!” type stuff. Symlinks are definitely more compatible outside of the Draft, no doubt about it, but they make very little sense in the Draft folder. Prohibiting them would mean prohibiting the feature outside of the draft on a piece-by-piece basis, since items in the Draft but located elsewhere by clone would violate the prohibition. This would lead to further confusion over why some things can and other thing cannot be cloned. Again, not a problem with collections. Anything can go into them because they exist in a transcendent position with relation to the binder.