Actually, it is considered Fraud and prosecutable to the full extent of the law. I'm always a "better safe than sorry" person, but we did nail someone for this at one hotel.
Our neighbors were having an unrulely, loud, sexual party and we know the hotel only allowed 4 to a room, but there were about 6-8 voices. So we called management. Security came and checked them our and by 1am, all of them were on the street with thier information already taken by the hotel. The one who made the room reservation is responsible for the settlement. The management did tell us that much. They didn't tell us how much the settlement was for, but probably for much more than it would have cost for them to get one more hotel room.
Granted, we only caught them becuase it was midnight and they were having a wild party.
But this goes back to the age old "If you find a wallet with $300 and ID on the ground, do you return it?"
You are also adding to the hotel's liability. If there was a problem and someone was injured in your room due to something going wrong (It's disney, but it CAN happen), Disney could not be held liable for your friends beyond the 2 on the reservation.
In the end, IMHO, it's just not worth the risk, but I'm a "play it safe" sort of gal when i travel.