If you need a unique identifier in form of a code or an ID number to your employees you can set employee codes.
This is how you do it:
Head to the "People" page
Hover over an employees name and click the blue edit button on the right
In the pop-up you will find one box for the employee code.
When exporting to excel, this employee code will now appear on every expense that relates to the individual members making matching to other applications much more easy.