The group usage therapy could be use by more persons, it could be used by 10 persons simultaneously with 1 operator. The CO2 operator execute it from a headfiller stage which is placed on a weighted car filled with 10 kilogramms.
It's advantages: the cheapness, during operation a good company could be gathered. Clubs, unions could do it in the same time.
A therapy's time is 40 minutes. During this time after 15 minutes another feeding is requires. So you can cure with two filling. If you feel that your pores need more fill you can fill more time, or you can ask the operator to fill it.