If you are the group leader, you will want to make sure that each person checks their own bag with the porter. If the motor coach company is doing the unloading of the luggage to the porters, make sure that each person stays with the motor coach until they see that their bags have been given to the porters and taken inside the terminal. It sounds like a hassle, but this is one link where bags tend to disappear and it will become your problem unless you have each person take responsibility for their bags.
In terms of tipping, $1 per bag is OK, but I would seek out someone to oversee that all the bags are accounted for and give them an extra $20 or so.
I hope that helps.