Hi Sandra. In addition to the answer above, you can also look at individual certifying agencies that offer group exercise instructor certifications. As around locally (e.g. local gyms and/or group exercise instructors) what certifications they hold or hire from, and this will help you in deciding which organization to go with if finding a job in group exercise is your ultimate goal.
By contacting or searching these organizations directly, you will also be able to discover which ones offer online versus in-person course work. In my opinion, if this is your first certification, I would highly recommend doing it through an in-person course since developing the necessary hands-on skills is vitally important in this industry.
I hope that this helps.