Use a cold brewing method, so your coffee won't be bitter. You can make vats of the stuff, and then re-warm as needed.
How big of a group?
Seems like gently warming a large volume of pre-brewed coffee might actually be harder than just brewing onsite. How did you plan to rewarm?
We've only done a couple of outside/offsite events, but in those cases have brewed at the store and done runs with fresh coffee as needed.
Looking forward to other thoughts.
do 2kg of coarse ground coffee to 15l of water, makes a double strength concentrate. i use paint strainers from lowes and stretch them over the lip of a 5 gallon bucket. i think you'd be better off just selling iced coffee since it's hot out anyways.