why don't you take 3 or 4 shots? You can shot 3 or 4 groups of people and then marge trough photoshop...It's very easy, and you need just 2 or 3 light points...
I did something like this in the picture below, first the shot for the people with one flash, then, with the camera still on the tripod the shot of the background that was huge...
(note that I have not finished yet the retouching...)