It seems unlikely that a corporate group would use separate rental cars for 43 people. They probably used a bus like most large groups visiting the tram. The question is whether they forgot to do a headcount or pass around a checklist-signout sheet, or if they were hoping that the missing couple would just magically show up back at the hotel.
Desert Sun article:
"They were part of a group of 43 conventioneers who decided to take the tram tour on Saturday."
"Authorities didn't launch a search for the couple until Monday evening because they weren't aware the couple was missing until Allen's sister called the sheriff's department."