I charge each client in the group a slightly reduced price from my single client price. For ever client that joins the group the price reduces slightly for each client. I make more for 2 clients than 1. More for 3 than 2 and so on.
I still spend a good bit of time designing the program so that it addresses each individual. This takes more time for more clients. And I charge more if the clients do not share similar goals and ablilties, as this also takes a good deal more time for me to design the program.
That you asked this question makes me think that you do not spend very much time in designing programs or focusing on each individual client. That would make me think that the trainer is not very professional or truly interested in their profession.