I'm surprised no one has mentioned QBox. It is specifically designed for this purpose. We've been using it to share our company files for two businesses between three computers and it works flawlessly. It only allows one user at a time to be logged into the company file so conflicted copies are a non-issue. Easy set-up and at $120/year, it's peace of mind. I have no affiliation with QBox, just a happy customer of over 3 years.
