All cruiselines pretty much have a 1 for 15 TC (tour conductor credit) which means for every 15 full paying passengers going the 16th gets the free berth. I am sure they explained to you that the free berth is a credit and on the cruise price only, not on port charges and gov. fees and taxes.
Is your group booked on Royal Caribbean or Carnival? The cocktail party on Royal Caribbean usually requires minimum of 50 guests depending on the length of your cruise.
I would advise you to contact your Travel Agent and ask where the other TC is going. If they are keeping it for themselves they should have disclosed that to you at the beginning.