Hey Steuss,
Girl Scout leader with Level 3 Outdoor skills at your service! I can offer you an immediate solution until you collect up umbrellas and/or ponchos. Our local search and rescue teams teach this to folks going up into the mountains. Go to Walmart and buy the biggest quantity of the cheapest large black trash bags that you can find. Don't bother with quality, no drawstrings. Get the kind you
tear off. Thicker is better, but in this case, you probably want as many as you can get for the $.
I'm just going to give you a link to a trash bag rain poncho image you can see, because I think the image is too large to post. This one has just a hole poked in for the face--that's what you need. No arm holes for rain.
http://masterwoodsman.com/wp-content/uploads/2013/12/TBsurvival6.jpgIf you like this as a temporary solution, you'll want to make one for yourself and any other adults on your volunteer team, so that when you guys go outside to pass them out
you can wear your own so folks don't feel like you're insulting them. Tell the folks to shake them off to dry them and fold them up to keep in their pockets until you guys can collect up something more suitable.
This works, I've worn them myself. Keep some in your car for when you need to do roadside repairs in the rain/snow or whenever you need a raincoat. ;-)
I'll expect to be going to Walmart tomorrow, will check to see if they have umbrellas and rain ponchos in stock and check prices. Many times they have things like rain ponchos at the checkouts.
Also, what about the foil blankets for warmth? Do you expect to need any of those? Quasi said something about wetter and colder...