shoot lots of shots really fast
i so much hate shooting groups, because the odds of a picture killing expression are squared with each additional participant. it's hard, hard, hard.
The only advice I've heard that I have found works for me at least a little is to sometimes give direction that doesn't make sense (and shoot really, really quickly, since everyone will focus for a second or two as they try to figure it out) or give direction that doesn't seem like direction, and again, shoot quickly to capture what happens.
I saw great pictures Jay Maisel shows in his classes about working with a group shot - it was his daughter's whole 8th grade class from a girls school, all together on risers, for a class picture. (He offered to do the shoot because his daughter was sick and missed the official one, and who could turn THAT down?) Somewhere along the way he was getting frustrated at the pasted on self conscious smiles and stiff attempts to look beoootiful that you get with girls at that stage of life. So he looked over the top of the camera and said, in their general direction, "you've got some hair out of place there, could you try to straighten that out". And then shot his butt off as every single girl reached up to adjust her hair (because every one of them worried it was HER hair that was out of place) and then they all burst into laughter at the absurdity of what they just did. Both of those images - 20 girls all looking aghast and straightening their hair, and all of them in unselfconscious laughter, were in the category of priceless.
I can't say I've ever found any thing as effective as that, but I try to be true to the spirit of it, and at least a good part of the time give instructions, directions, or converse about things OTHER than having their picture taken.
From reading all of the "how this picture was done" in the photography mags, it seems that a huge percentage of the group shots we see these days are individual shoots put together in photoshop afterwards - the priceless Sopranos images, a few of the "here are all the supermodels who worked on this" shots, even the group photo of the VII photographers in the Canon ad have two people photoshopped in. If I'm really awful in this life I suspect I'll be reincarnated as a groups photographer.