DME is an individual thing, not a group thing. You can come and go on 4 different flights on 4 different days, it doesn't matter. Just make sure the room reservation reflects the correct days of arrival and departure for each person.
As long as each person is named on the room reservation at a WDW owned-and-operated resort, then each person is entitled to use DME.
The DME CMs will do a better job of arranging all this than the Disney Reservations Center CMs. However, DME doesn't have access to your reservation until 60 days beofre arrival. You could wait until that 60 day mark and call DME directly (866-599-0951) to make your DME reservations, to make sure they get done properly.