I don't think Dropbox was ever intended to be a file sharing tool for multiple users. It is a backup tool that also keeps all of your computers synchronized. Since I have a collection of a primary laptop, netbook, HTPC, and Android smartphone, it has been great at letting me access any of my documents from anywhere. Even if I was on my work machine, where I don't want to store any personal files, I can access one if needed through the web interface.
Sharepoint is a great tool for document collaboration, but can be quite the investment of time and money if you don't already have a server setup.
Google Docs or the other cloud services may work for you, but I have not played with them much.