Ok. So, the guy that saves everybodies life is a man named David. He tells them all something along the lines of "Come with me, or die," and then guides them to his van. If they get in, they live. Simple. Nobody knows anything about the conspiracy, or anything in detail about David. Only that he is a very normal looking man.
The IC will be up soon. Your first post should look something like this.
How your character saw the events on the night of the prom. Why they weren't there. Why they were somewhere else, on the outskirts of town. What they thought when they realised their family was now dead. -> Then, they run in to David. He guides them back to the van. -> The van trip, which is mostly silent. -> Arriving in the forest, getting kicked out of the van with limited supplies and watching David drive off.
After everyone finishes this opening IC post, it will become much more free form, but try and make your intro something like that.