Cat herding is hard. Consider being the motivational speaker at the start, making sure everyone has their witness forms or knows what to do with photo capturing their receipts with their odo, and has practiced doing that prior to the ride, remember you can't witness each other, so if you're doing it old school on paper with witness forms, (arguably simpler for a group), make sure they know they can get the odo witness the night before you start.
At the start gas station you choose, double check that everyone has their forms signed for odo verification on the start odo. Doesn't matter if they did it the night before or at the gas station, but make sure no one forgot. Remind them all to get and keep their receipts!
After that, honestly, don't see them again until you get to the finish gas station. You are not a group. Do NOT stop because someone else needs a rest stop or gas when you don't. Do NOT linger at gas stations talking about the ride or you increase your chances of not finishing it.
Reinforce with your group at the start that it's a solo challenge. Stay focused, stop for gas, get gas, get receipt, get GOING. And it's HOT out there. Hydrate or DIE. Getting dehydrated means your brain is slowing down and you make poor decisions. A lot of small sips is much better than big drinks at gas stops. If you think about drinking, take a drink, that's your body telling you it needs the fluids.