Oh man! First of all, I would let your group know that they are responsible for their own luggage. Make sure they individually check it with the porters. I would suggest a $5 tip per cabin and I would have each cabin in your group take responsibility for both their luggage and the tip.
The last thing that you want to happen is to have your group get off the motor coach and head into the registration area without making sure their luggage is checked with the porter. I know it sounds harsh, but anything less than this will lead to a disaster for you.
Just my opinion.