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By default, when a drawing is saved, AutoCAD does not save current user's name with the drawing as file property. Therefore, you have no way to know who is the last user who has changes being saved in the drawing unless you do something during the period when drawing is open in AutoCAD: when drawing is opened, saved and/or closed...
The obvious option is to use drawing custom properties (DatabaseSummaryInfo in .NET API, there is eqivalent in VBA, too). You can save user name in DatabaseSummaryInfo whenever drawing is (to be) saved or to be closed.
Of course in this case, you can only read the "last user" information when the drawing is opened in AutoCAD, or at least when AutoCAD is running and you load the drawing as side database.
If you store the drawing is some sort of document management system (such as something based on SharePoint), then there must be some kind of custom saving command, which can save more file meta information, including who is the last user, together with the drawing file. Then you can get these kind of meta information without opening a drawing file in AutoCAD.
