Canon 20-35mm f/2.8 L?

You will have a bit of a gap in your "armoury" between 35mm and 70mm - a lens of around 50mm is really useful for portraits on a crop-frame of 1.6 ratio. There is almost certainly a lens in the Canon stable which is 17mm to 55mm (or to 50mm?) at f2.8, which will give you what you want and a better chance of portraits at 55mm and a somewhat wider facility at 17mm if the big group is in a small room. That lens is specifically designed for crop-frame for the sort of applications you want.

I have a feeling that the 20 to 35mm is aimed at full-frame bodies, where it would supply almost all wide-angle needs for many people using full-frame (but would not be really a "standard zoom".)

The good things I see with the 20-35mm f/2.8 L are the bigger aperture size on the 35mm end and more importantly, better optics. Otherwise, the 18-55mm kit lens would be more useful especially with portraits. If you are after faster shutter speeds, get those two prime lenses instead.

I owned this lens when I shot film. It was a pretty good lens, but not as sharp as Canon's standard and telephoto f/2.8 L zoom lenses. The newer lenses from Tamron or Sigma with newer lens designs could possibly be better. While photozone.de doesn't have a review of the 20-35 L, they do have a lot of others all with sharpness charts - http://www.photozone.de/canon_eos_ff

I have one and I love it. I really like the a7s though too its SO much lighter rand easy to carry