What else will you use this "team photo" lens for?
I am biased, and this is just my opinion, but I have taken kid team photos, adult coaches on the side, trophies in the front, and NEVER had to go wider than 24mm on a 1-D MkII. That's equal to 32mm focal length field of view. On a 20D, 20mm focal length lens would provide similar field of view.
You can do that with Canon's 18-55mm kit lens. Unless you're also going to shoot landscapes, street candids, or other things where you demand high quality wide angle, the 17-40mm f/4L seems expensive. In team photos there is always a kid picking his/her nose, closed eyes, etc. I don't find sharpness "the" problem for an f/8-and-be-there photo to make parents happy.
From experience, I feel a 28-XXmm lens is not wide enough for teams on a 20D. And 40mm is not long enough for sport portraits. But, if you can back up just a couple feet, and capture the team with 24mm on a 20D, you are likely to get far more use from a "wide-normal" zoom like a 24-70, 24-85, 24-105 IS, or Sigma equivalent. My Canon 17-40mm, while a nice lens, is my least used. The 24-70mm is most used, even on the 20D.
If you also shoot landscapes and whatnot, the 17-40, or Tokina 12-24 will serve you. If "teams" are your only reason for 17-40, well I'd back up three feet and shoot from 24mm... But, that's me. Jack