I have been asked to shoot groups of up to 20-30 in what could be dimish lighting. I have my 430ex and was contemplating renting a 16-35 (or 24-70) and 5D. What are your oppinions? I might be able to couple the 24-70 with my hope of an imminent purchase...but will that be wide enough on a 5D. The room is not super large, so that's another thing to consider.
Beware of perspective distortion which can easily accompany a 24mm on FF or 15mm on APS-C.... If you cannot stand square to the group, those closest to the lens are more accentuated in the photo in their prominence in the shot, so that you can make the fat lady even fatter and the petite gal far away even more petite, so the fat lady looms even larger than she is (and guaranteeing you her wrath!)
I have covered weddings quite well with medium format, using lenses no wider than 28mm (35mm film equivalent), but I will admit that I have not dealt with 30 people in a group! I would suggest having tiers of 10 per tier, to minimize group width