Firstly, I'm assuming you're referring to the 5DII rather than the 1DII, in which case, I identify! The flakiness of the outer focus points in dark and challenging situations is well known. I have both the 5Dc and 5DII and have long since learned to fall back on centre point 'focus-and-recompose'. Second, I don't know your level of experience - if any of the below is obvious, feel free to ignore . . .
If it's a single-line group and you're shooting wide angle and have to shoot wide open, you may want to arrange the line in a shallow arc so that all parties match the field of curvature and all are in focus. If you're shooting tight on the group, be aware that wider angle glass will horizontally 'stretch' the peripheral figures.
If ambient light or flash permits, I usually crank down two-three stops from wide open to give best IQ and increase DoF.
If it's a two-line group, I'll try to focus on a high-contrast ear on the front row - giving the 1/3 front, 2/3 rear DOF ratio a chance to bring both rows into sharp focus.
Three line groups are simpler - just focus on a good contrast area in the middle row.
Don't be afraid to bump up your ISO if you need to stop down for decent DoF; the body gives excellent results at 3200 and is quite acceptable at 6400 in decent lighting.
Hope that helps.
