I don't have experience with any of those programs. So let me suggest another one. That's Windows Sharepoint Services that's including in Windows Server 2003 and higher. If you already have a server with that OS, it's might be worth looking into.
It allowes for shared storage with checkout-checking checks when updating. So only one person can update a file at the same time.
In my office several people need to have access to a few files. I understand there are lots of options out there for auto syncing these files across systems.
My question is how good are Groove 2007, Dropbox, and Expresso respectively? Are there any other similar services I should be looking at?