More Personal Attention. Working out in a small group allows the personal trainer to offer corrections and appropriate modifications and progressions.
Variety. The workouts change and evolve as the group progresses.
Motivation. Having someone exercising right beside you often encourages you to keep going.
Support. Both the trainer and the other participants are there to help you to get through the challenges in front of you. You're all on the same team!
Fun. Yes, fun! The right exercise program done with friends can definitely be exciting and enjoyable.