Coming from a group instructor (Rebel), yoga teacher as well as a personal trainer background, sometimes I wish that more people would start off with coaching from a personal trainer before jumping into the big ones. This is not to say that one cannot improve by joining a group fitness class, but perhaps for some, starting off with good coaching and a solid foundation will reap greater benefits in the long run.
Many times, people join classes with incorrect gait, muscle imbalances and lack of body awareness, which might in the long run cause injury. Lordotic posture, collapsed knees, kyphotic backs, all these could lead to muscle, or worse still, joint injury. And sometimes one-to-one is the best option.
This is mainly because any group instructor can only stay with you for a maximum amount of time. Keeping an eagle eye on every single person (especially if it is one instructor to more than 12 persons) is a bit risky. And at the end of the day, accidents to happen.
So if you are already injured (ankle, knee, shoulder etc) or if you have a pain when you exercise (shin pain while running, low back pain after running etc), then my suggestion is start off first with personal training. And choose your personal trainer wisely. Certification might not be the be all and end all. In fact, sometimes experience counts for more (as opposed to a freshly certified instructor).
Of course, good references by way of word-of-mouth is also good. So maybe think about this before you begin your journey in group fitness classes. It might make the difference between a permanent injury and massive success.