Hi,
Welcome to POTN
IMHO you have just described the Canon 17-55MM f/2.8 EF-S IS. It works great for large groups and for weddings and I use it on my 30D. I have used it in both situations with great results. Cost will be right at $1,000.00 so this is not a cheap solution.
One thing on the large group shots...get a small step ladder or 3 step kitchen stool and shoot down to the group...shot this way the pictures will look great.
The POTN folks here are super...in the next several days you will get lots and lots of great advice...so, read it all, apply the advice to your situation and your wallet and make your decision.
Was kind of leaning that way, however was kind of hesitant at the price but was thinking I would get a lot of use out of it. Seems like this is the a popular lense. I need to get a few other things such as a bigger memory card, and I have not purchased a light meter yet either. On the other end of things I did consider the Sigma 24-70mm f/2.8, but am pretty sure I would still be looking for something better in the long run.
What's wrong with the 50mm f/1.8? I used that for my family portraits (18 members, 6 per row). I'll admit that it was a bit long for a crop camera and I had to setup the tripod quite a distance. I shot outdoors at noon with overcast sky. If anything, I'd recommend 17-40 f/4L or a 30-35mm prime.
With the 50mm f/1.8 I have read alot about focusing problems. I am worried about a group this size. What if the middle row is all in focus and the back row isn't? Major screw up. I have it with three people where one of them is out of focus. Seen as how I am still in the learning process, it more than likely is the person behind the camera then the lense itself. I probably need to play with the aperature on that lense as I will on any lense. For a group this size I should have my aperature set between f/8 to f/11 or so, correct?
As well with the 50mm I am concerned about how far I would have to stand back. Too bad I didn't have a trial run at this.
As well, the 17-40 probably will not be as useful with other portraiture. At least I think that is right if I am understanding my lenses correctly.
Ever lay a wake a night worrying about these things?