50mm f/1.2L for taking group pictures
In bright daylight this shouldn't be a problem at all. Just stop down as much as you like - maybe f/11 to f/16. In low light it might be good to set manual and set shutter speed to 1/50 and and start at f/5.6, of course the deeper focus the better if there is enough light. For group shots on the 50mm you'll have to stand back a bit anyway - this should give you more leeway in terms of depth of field. Set ISO to auto in such a situation and allow it to go to 6400 or even 12800 if absolutely necessary. 6400 looks pretty good on the 6D; 12800 not quite as much but it's still better than out of focus subjects. I tend to avoid 25600 but many people (especially with a 6D - I'm on a 5D Mark III which isn't quite as good at high ISO) are fine with it. If all else fails move them inside and bounce flash off the ceiling.