I've set up several groups with a Windpro & Open Country 4qt aluminum pot combo, should be able to cook for 6 w/the pot... think the pot is ~12oz, stove ~7oz + ~1oz for wind screen.
That said, I wouldn't discount the Reactor. It's the most fuel efficient canister stove I've used, getting nearly double the fuel efficiency of standard canister stoves I've used. Fuel weight's not a small deal when you're heating that much water! If you used Reactors w/the 2.5L Reactor pot, three would get you all done at once. W/two of them for 12, one group of people would have to wait a relatively short turn... no big deal.
Oh, probably figure ~2C/500ml boiled per person per meal.