Is there any way you can get out of doing this shoot?
Seriously, it sounded only difficult until you said you might only have 5 minutes setup time. Pressure like that is bound to make you forget something and/or otherwise screw up.
Others have given you good suggestions, so I will just reiterate the ones I agree with:
No bare bulb, unless you put a diffuser in front of it and a large reflecting surface behind it. A portable reflector, or even a big sheet of white foamboard will suffice. Alternatively, a large diffuser in front of the flash. But either will cut your light by a couple of stops or more.
You really, really need a flash meter. An auto flash will read the dark background and overexpose the people. An ambient flash meter will give you the right exposure in seconds, and let you check to be sure it is even across the spread of the group.
An 18mm is a really bad lens for this. If you use it, put the fat people in the middle and the skinny ones on the ends. Or put the people you like in the middle and the ones you hate on the ends. Really. I never use more than a 35mm for groups when shooting 35mm, and even then, I try not to fill the width of the frame.
But really, consider a change of venue.