How do I track who uses my Excel spreadsheet?

How do I track who uses my Excel spreadsheet?

I created an Excel spreadsheet that my boss wants to put on the company's
internal website. The spreadsheet contains some seldom-used, esoteric, but
handy functions that only certain employees within the company will find
really useful.
The problem is that I don't know who the future users are, and my boss
wants me to identify who uses my spreadsheet.
He asked that I password-protect the Excel spreadsheet in such a way that
one password does NOT unlock all of the copies that people can download
from the site. For example, I can't just make the password "stackoverflow"
because once a user legitimately gets the password from me, and is shared
with other people, it can be used by anyone within the company to unlock
all subsequently downloaded spreadsheets. I will never be able to
ascertain who is using the spreadsheet. Also, I cannot modify the website,
so I hope to achieve this tracking of users through Excel and email.
Is there a way to have Excel randomly generate a string, which the user
emails me, and then I respond with the appropriate password that will
unlock the file (based off the generated string)? This requires the user
to check in with me before using the spreadsheet (the ideal situation).
Is such an arrangement possible in Excel 2010 Professional Plus?