Tim - I'm in! I've got three Holgas of my own and love what they can do. My suggestion for speed is 100 if most people can shoot on sunny days, or 400 if most people will be shooting on cloudy or overcast conditions. I know that's a little hard to predict. For me, my go to speed is 400, but that's just me. Also, I would prefer black and white, but I wouldn't be opposed to colour either - maybe everyone in the group could vote for their preference?
If you're doing a round with 12 people, wouldn't the 6x6 format make more sense? 12 shots for 12 people. Otherwise the 645 x 2 shots for 8 people works too. Again, just me, I prefer the 6x6 format since you really get the full Holga effect with all the weirdness in the out-of-focus areas, and some vingetting as well (especially if you remove the mask entirely and tape up the edges on the inside of the camera). But I'll go with whatever comes my way.