You recall what shutter speed was used? I'm guessing it was on the higher side, causing the background to go a bit dark. Dragging the shutter around 1/30 would let more ambient light come in and improve the overall illumination of the shot. It would certainly lighten the background.
You could possibly pull it off with a 24mm if you made two rows, or having a bit more profile of the individuals to shorten up the line. Another alternative is to bring a few chair in and make a "front row" for about 4 people to sit. This too will shorten the line.
Here's one shot with the 6 megapixel Digital Rebel at 21mm. The most glaring problem here is the on-camera flash which just didn't have the GN to do the group justice. This is a pretty big group shot so I'm thinking 24mm might work. Then again, I could certainly be wrong

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