I would use your 50mm @f5.6 or about and bounce the flash off the ceiling if you can. You didn’t say how many rows of people you are going to have and if you want full body pictures. You might want to shoot down on them just a little so no one is hidden from view. If the 50mm is not wide enough and you can’t get back far enough to get everyone in, then use your 24-70mm and try to keep it as close to 50mm as you can. If your ceiling is white and 8-10 feet high your bounce flash will work great. Get one or two people to take a couple of test shot before you get the group together so you’re ready to go when they are. Have fun with your shoot.