I just got off a chat with one of Dropbox's representatives.
In short, I have a Dropbox professional plan and I created a separate folder for each of my students. Yesterday, one of my students got a message that her account is "full" despite me having two and a half terabytes of free space left. Well, guess what, space is calculated double - once on my account and once on hers. So, let me get this straight, if I share a big folder with a lot of files with, say, 20 people, Dropbox will require all 20 of them to pay for the same amount of space!?
Anybody from Dropbox cares to justify this!?