Word and Excel file users

I need to be able to know all the users who accessed the word and excel files for a particular job.
The people on this job are using Office 2010. The files are stored on our network.
For example, if Bob made a change to John’s document, then I should see both John and Bob in a list of some sort as users who made saves to the file.
Because this is being installed now, I don’t expect it to work retroactivlly (although that would be cool).

Many thanks,