Check out military and 'boot camp' training methods. While they may not be the best ways to train, it is an effiecient way to train a large amount of people with little or no equipment.
It would probally be best to split them into two or three ability groups. A problem with mass training is a workout has to be geared to the weakest person. While the ones who are out of shape are struggling just to keep up, those with advanced fitness are wasting thier time.
What type of equipment do you have available? What type of company is this? What is the general fitness level of the class?
Maybe the biggest challenge is to keep motivation up and to keep things interesting. Variety helps, as well as team competions. Good luck!