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.