The rich blonde girl (the one that supposedly went to France) ending up staging the first murders (the ones that show up on the video camera they find in the woods) with the guy that she supposedly was going back to the cars with. Remember when they asked the guy "don't you want the $10,000" and the rich girl says "I will pay you $10,000 to get me out of here". Those two were the ones picking off the rest of the group and killed the real Shep and Shep's girlfriend before the move even starts.
You find out toward the exposition at the end that Andrew, who is thought to be cousins with the rich blonde girl is actually BROTHER & SISTER therefore the rich girl was essentially trying to kill her BROTHER and the rest of the group to make it look like a big murder plot so she would get the vast family fortune. The rich blond girl has actually killed her parents before they left for the camping trip and never went to France at all but was in an insane asylum that she broke out of.
Near the very end the rich blond girl is killed (or at least we think) however when the police do the investigation there is some discrepancy about how many bodies should be there.
Several months later, Andrew is at home with his girlfriend (yes she makes it out as well along with Bob) and Andrew gets another note from his GPS website from Shep (spooky eh?). While he is doing this he gets a very odd knock on the door from several folks looking for a lost cat. This distracts him just long enough for a man\woman wearing the same black outfits using during the murders to run upstairs and the last thing you hear is the girlfriend upstairs screaming. Its unclear if it was the original killing pair or a third accomplice.
They also find Shep's fortune as it was written on a piece of paper that was used to roll a joint\cigarette.
Thanks Greg O!