Again, I'll reiterate the answer 'depends'. We have no idea how close you will be to the group. Distance is the key factor here, as well as your perspective in relationship to them.
If you're relatively close, you'll need an ultra-wide. If you can step back a bit, you might just need a wide-angle. You may find a 50mm is perfect. If you're some distance away, a 105 or so might work best.
Without any further details, we're just shooting in the dark here.