I just took a family portrait yesterday; all 19 of us after Thanksgiving dinner. I used a 28mm lens on a tripod, set it up so that everyone could easily be seen, and then I cropped out the bottom edge.
I find a 28-35mm focal length to be about right for full-length portraits of adults in groups. Those lengths are wide enough to get everyone in without introducing optical distortion that can render people out of proportion.
With high megapixel cameras (anything over 12mp these days on a DX or FX sensor) cropping gives up very little in quality. I like to set up the shot so that everyone looks good and then anything extra in the frame can be cropped out later.
