[MrB]

Faye, the primary important factor here is Depth of Field, as you have quite a distance between the nearest and furthest people. This requires that you use a small aperture (i.e. a big f stop number), so use Aperture Priority Mode and set e.g. f/11. A lens in the standard range (40 to 60mm) should give a natural field of view on a full-frame camera such as your Canon 5D, while not making the furthest people look too small. A prime 50mm lens wouild be best , as the sharpness at the edge of the frame is usually better than it is with a zoom lens. As Ed has written, start with the ISO at 800, but check that the indicated shutter speed is at least 1/60 of a second. If it is slower than that, increase the ISO - on the 5D you should be able to go up to 3200 if necessary, and still get acceptable shots.

Cheers.
Philip