Water treatment for 15 people

Miox, while expensive, works well and can dose out to the size that you like. CR123 batteries are a buck a piece at Amondotech dot com. Rock salt is cheap where you can buy ice cream makers. You make the solution as you go.
The Katadyn Vario is a filter with a high flow rate if you bypass the ceramic wafer (an optional way to use it). It is bulky and is still a pump.

Polar Pure is definately the way to go. It's cheap (~$15), the iodine crystals are large, and it replenishes itself. And I think that you can find it at REI.

http://www.polarequipment.com/

If you can't live with the iodine taste, use chlorine dioxide and at 1/2 strength and let it sit for twice as long.

Ted

Katydin Micropur tablets. IMO, as a group leader they might be totally trusting your judgment, so don't use bleach or some 1/2 measure. You need to use something which kills viruses, bacteria, giardia and cryptosporidium. Explain it is expensive and charge them the 43 cents per liter.
Just my opinion as a former and current small group leader. It can also be used at 1/2 strength for twice as long, but in warm water it works much faster than the 4 hour standard.

For your case, IMHO, Gravity filter is the way to go. I have the Amigo H2O from ULA Equipment. There are also a couple of MYOG articles for gravity filters in these forums.

The have the new Reliance PUR Chemical Treatment product on the home page that they love if you are treating for 15 people.