At the moment an external user I share something with gets a clunky, non-intuitive browser experience. The thing needs to look pretty and be functional. Strip anything not needed and make it look more like DropBox. You could have admin options for what an external user sees, or you could have relevant browser tabs visible depending on what has been shared.

When we add the ability to share folders externally, the user could see the folder(s) AND what has been shared in a single window, then navigate through the folders.

Bottom line - it aint pretty just now!