Almost everyone will come in late.
It might be best to begin on time, and to close the doors when you do.**
It might be better to leave the doors open.
In either case, an explanation is helpful.
*Wikipedia: The Phantom was the first fictional hero to wear the skintight costume which has become a hallmark of comic-book superheroes, and was the first shown in a mask with no visible pupils. At its peak, The Phantom was read daily by 100 million people.
**e.g. on a train journey