Without the backpacking group knowing, I was keeping tabs on the water levels. We brought in 2 seven gallon containers plus 2 cases of bottles water for a total of 19 gallons. From Thursday 3:00pm until Sunday 11:00 am, the group drank 16.5 gallons. This is non-rationed of water. It included drinking, cooking, and cleaning usage.
Total hours: 68 hours. 16.5 gallons by 2.8333 days = 5.8236 gallons per day.
This proves the old adage of 1 gallon per person per day. We did have a few other adult beverages with us too. 1.5 gallons would be a good rule if no other liquids were available.