A lot of companies and government agencies cannot use online services for security/privacy reasons. THen, your options are basically use your own server and have users log in as Steve suggested. If you use online service, users need to take into account the time it takes to copy from the users hard drive up to the cloud storage, and then back down to the other linked computers. Not a big deal, but it is not instantaneous. If you want to use a third-party system for online storage take a look at Microsoft OneDrive. For only $99 US, you can subscribe to Microsoft office and share 5 installs of office in both Mac or Windows versions. More importantly, you get a full TB of storage per shared user (or just use one central one). It works basically the same as Google Drive, Dropbox, Box, etc. I use all of the listed services, but OneDrive gives me the most value. If needed, these services offer higher security options for extra cost.
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