Larger groups require wide angle. If you're using a crop body, which I assume you are, I'd shoot with something like the 24-105 L. I've also heard good things about the EF-S 15-85 as well.
It really depends on how big the group is and what the lighting conditions will be. The 24-105 if f/4 and is good in most situations, though being "only" f/4 it will be a matter of raising ISO to get a good shot, so if you're camera is good at 800 ISO, this would be a good purchase in my opinion.
24mm on a crop body is 38.4mm on a ff camera. That is not very wide at all.
Consider stitching photos together in post, or get a wider lens.
Without know your budget, current lenses, and shooting conditions here are some general recommendations:
If you can provide us with the lens, approximate subject distance, and depth of your group, we can provide you with some aperture settings to ensure adequate DOF. By their nature, wide angle lens produce ample DOF so it really shouldn't be a problem anyway.
If you are planning to be in the shots, pick up a decent tripod and remote to trigger your camera's shutter button remotely. The RC6 remote is very inexpensive and works excellent. It has a two second delay switch allowing you to press the button, then move your arm back down before the camera fires.
I'm assuming you are shooting in jpg. Learn how to enable exposure bracketing which will allow you to take 1 shot at normal exposure, one slightly under and one slight over. This will give you 3 pictures at different exposures. With the remote in your hand and bracketing enable, you take 3 separate shots in a row and you will end up with 3 images at different exposures so you can pick the best one.
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