This is cumbersome when trying to share multiple documents. One has to attach everything before sharing anything with the students. The video above says that this is intentional on Google's part. I can't see the benefit.
I'm still really liking the tool. But I'm starting to find interesting glitches.
update: you don't have to start from scratch. Thanks +Kyle Beatty for showing me "reuse" post. This will still allow you to make a copy, even if you didn't have it as a copy in its previous iteration.