Very hard riddle

There is a hotel. Inside the hotel are 100 guests each staying in individual rooms by themselves.
Before all these 100 people check into the hotel the security guard tells them that there is one room in the hotel that is empty - all it contains is a light switch that you can turn on or off.
Every day he will come into someone's room and bring them to the light switch room- where the only thing they are aloud to do is either turn the light off or turn the light on.

The last person to enter the room must announce that he is the 100th person to have entered the room.

Here's the catch - the only time the people are aloud to interact is before they check into the hotel where they are aloud to devise a plan amongst themselves that some how the last person will know he is last.

Then they all check in and are no longer aloud to communicate.

What is the plan?

(They aren't aloud to leave belongings in the room and the security guard has nothing to do with it).