Officially - you can't do this but if you are okay with the perk limitations and the honesty that is involved - there really is no problem not listing everyone on the reservation.
Hold it!
I've worked at three strip properties here in Las Vegas and while it isn't Orlando, if you are an unregistered hotel guest anywhere, there can be HUGE ramifications if there is ever a situation where you need the hotel's assistance.
If there is an emergency (fire, etc) and the property is evacuated and you have to leave stuff behind, like suitcases, handbags, medicine, tickets, IDs, cash, etc - you do NOT have the same status as a hotel guest with respect to getting these items returned to you.
If you are injured and the hotel is at fault and you are an unregistered guest, same thing. The hotel does not have the same level of responsibility to you as they would to a registered guest.
I would NEVER try this, ever, no matter how much money I would save. I worked at a property (Bally's) that had a fire in 1980. I worked with the person who had the responsibility of returning luggage to hotel guests . . . and also, those who were not guests. What was described to me was the most giant, unforgiving responsibility and I had the utmost respect for what she went through after the fire to make everything right again and return guest's luggage and property to them. For this reason alone I would never lie about how many people I had in a hotel room just to save a few dollars.
What happens if you get "found out"?
Probably nothing. People do what you're describing all the time. If hotel operations at each Disney Property checked every room for how many people vs bedding, then they'd never get any work done. But it's not something I would do, simply based on my experience working in the hotel industry.