Time to Party

Lord Voldemort has just took control over Hogwarts and wants to throw a party. $N$ people are going to attend the party and they have to invite themselves .Each person has a unique number (which we call his id) between 1 to $N$. They have a strange way of inviting people .The process is "Every person invites those people whose id is a multiple of his own id by sending them a balloon and he can't i