Legally, no requirement.
Practically, a group-ex certification is important.
They’re vastly different skills. I started in group-ex and moved into personal training, so my experience is in the reverse of yours. In group, you have to be able to oversee and manage a group of exercisers who may or may not be at the same fitness level. You are constantly scanning the individuals, you need to have progressions and regressions planned for almost every move, and your instructions have to be very clear (in most cases there isn’t time for tell / show / do), and you have to plan a flow. Also, there are rules (aka “contraindications”) that are in place for group classes – things that a certified instructor would not do in a group class because we don’t have time to assess them and see a Par-Q.
The good news is, since it’s bodyweight only, that you won’t have to worry about switching in and out of handweights etc.
People can be good at both. I am. But if you’ve never had to manage a group class before, it will take some planning.