Serious post now, what I did, for I think like 6 months with my team, was used Dropbox. But I never installed Dropbox, I always uploaded files from the site, so the file being uploaded was never at risk and the original file always stayed on my computer.
And I'm assuming by Dropbox, you're referring to any cloud? Because from what I understand, they're all susceptible to sync errors (corrupting).
Dropbox corrupts it if you open the map file in dropbox and save to it. I don't know why.
What I've always done i keep the map in Dropbox, and whenever someone works on the map they copy it to their own folder outside of dropbox.
When they make changes, they rename the file with their name and the date and upload it. This means that no one's work get's overwritten on accident and it's not too hard to merge files or keep track of what was going on, and none of the files are corrupted.