By Invitation's upgrade credit program is ending on December 15th. Here is the full text of the announcement:
Our Promotion Is Ending
Our By Invitation promotion is ending on December 15, 2013 and it will no longer be possible to earn upgrade credits as of December 16, 2013.
From January 6 - December 14, 2014 you will be able to redeem your credits to upgrade your stay. You will also receive complimentary basic Internet, late check out, and exclusive offers in 2014.