that the first time they enter the room while the light is off to remove their token and leave the light off,
the second time someone enters the room while the light is off to turn it on and deisgnate himself the role of counter.
then for the rest of that period the light remains on, to signal to the others that a counter has been chosen, and to ensure there is no error. Therefore on the last day - say day five, there are two posabilities. Either the light is on because the counter turned it on some time before, or it is off because there is no counter.
If it is off and it is your first time in the room, then you know everyone else was in the room. If it is off and it is your second time in the room you know one person is left to enter the room. if it is on you don't know how many people entered the room, but you know the counter counted all the ones to enter while the light was still off.