Ask HN: Revision control for non-hackers

I am project manager for a university textbook project. The book is being written collaboratively, with some authors in the US and others in China. How can I manage the various documents and revisions without using Git, Mercurial, or some such hacker-friendly tool which is most certainly beyond the grasp of the authors? Would a service such as Dropbox work? So far, they have written the book in MS Word format, but I intend to migrate to a TeX-based system for typesetting and layout. Ideally, I'd like to use something like LyX, but online and collaborative. Does such a tool exist?