How you can help until a rehabber is contacted
First thing DO NOT feed the animal. If they are dehydrated, feeding them will act like a poison and kill them.
Second thing they need to be warmed up, use a heating pad on low under their box. Or place a soda/water bottle filled with hot water in the towels with the contained animal. These little ones are use to sleeping with their mom, she keeps them warm.
Third thing they can carry parasites, fleas, mites,etc. These little critters don't care who they jump on. So while handling wildlife you might get a gift also. Be careful!