Hubby, kids, and I are arriving and staying @ POR for eight nights, but we arrive one day before his daughter (17) and brother (adult) arrives (on separate flights). Will they allow my husband to take the Magical Express TO the airport to meet her/them and ride back on the ME? I did put the dates and airline info in the reservation.

I don't know if they would allow that. DME is just round-trip transportation, not double-round-trip. You could ask, though. But they to scan your transportation voucher at the airport when you board the bus, and you surrender it when you go back to the airport, so you would want to make special additional arrangements, not just have him get on the bus with his return voucher (or he might be stuck in the airport!)

Although I'm older than your husband's daughter, I would feel funny if someone rode the bus 2 ways just to meet me at the DME counter and ride the bus back (plus, she'll have her uncle) - but I WOULD be disappointed if Dad wasn't waiting at the resort at the drop-off location for me.