Personal Training and Group Training are two different types of beasts. While there is overlap between the two, the group training focuses more on choreography, cueing techniques, group safety and performance as well as modifications for different intensity levels. There are some people who are inappropriate for group training due to their health issues and you need to be able to screen that out quickly. As such, many gyms require group instructors to have a group training certification.
There are many certificate programs for group and personal training. If I were starting out (as a fitness pro) and did not know what to do-I would get a personal training certification from ACE, WITS, or NASM and then decide if I wanted to teach individuals vs. groups.
Hope this helps.