You did pay for the book you pre-ordered, right? Or at least had the courtesy to call and cancel your pre-order?

You reserved a copy knowing that there might not be a party. There was, so you thought Borders should change their rules just for you and allow your guest to also get a book -- in front of others who also did not reserve their copy in advance.

Those additional 500 copies that you were told about were for the people, like the person you invited, who did not pre-order their books. You want special privileges, sorry, but congratulation to Borders, they won't let you "cut in line" ahead of others who planned ahead and reserved their books.