I find that it's very helpful as a group leader to stop at every easily-accessed water source to ask who in the group might need to gather and treat some water. This is especially important when water sources could be an hour or more apart. On my trips I've been encouraging my groups to carry just what they need to get from water source to water source (of course, only where feasible, and building in some conservatism)so that they're not lugging 3L of water with them when they don't need to. But people won't always check or speak up, until they're all out of water and you've passed the last water for 5 miles, 10 minutes back. Best of all is to try to coordinate so that multiple people are filling up at the same times. These become natural rest breaks.