Not a dumb question.

To know who is opening a spreadsheet, you'd need to use macros (VBA) and the person opening it would need to enable macros. So probably not a great option unless it's already a macroed workbook
To track changes on a spreadsheet, you'll need to save it as a shared workbook. Then on the Review tab, "Track Changes>Highlight Changes. I prefer checking "List Changes on a New Sheet" and uncheck everything else. Excel will create a nonsavable worksheet showing who made what changes.
Does that help?