Interesting. That's the first time I've seen an online DME form that allowed as much control over flight info as that does. It might be a bit too involved for someone who isn't very computer savvy, but you can create "flight groups" and add different people to different groups as necessary depending on when they're flying in or out. It just takes some thinking out how to group people if their itineraries are complicated, like mine will.
Try this on for size: 6 people staying at SSR for a 7-night reservation. Two don't need arrival DME because they'll be transferring from another resort but will need transport on checkout day. Two arrive on checkin day but leave on day 5. Two others arrive on day 4 and leave on checkout day... And at the moment I'm not even sure if everyone will be on the same flight as the other half of their pair...
-Rob