I told them what I was trying to do, that they needed to sit in the same focal plane, and that if I looked at them from the side, I should only be able to see one person. The girl on right had a tendency to turn her body for portraits, which I had to talk her out of.
It was a bit of an experiment to see if a group shot at 1.2 with some distance can work. I usually play it safe with group shots and shoot at f4.0 and above, which I'm starting to re-think. Its definitely worth the risk and extra effort to aim for a larger aperture.
Like yourself, I take all group photos wide open at f/1.2 (or f/1.4 for 35L and f/2 for 200L)
The other way to do it when you can't put them into the same plane is to focus stack (this is helpful when it's awkward to put everyone in one straight line)
Also when in doubt, it's better to focus for the person in the front, rather than at the back (though I always focus stack just incase), as it is more obvious a person is OOF if they are in the front row
I don't take large groups at 85 f/1.2 usually, I prefer the @ 200mm @ f/2 as it gives a bit more DOF but blurs out the background more
(the below photo has not been edited, but for a larger enlargement I would focus stack the guy leaning forward so that he his tack sharp too):
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