It really depends on the ages of the extra guests. Some hotels like Circus Circus, don't charge if you are under 18. Assuming you all are over 18, then you will be taking a chance by putting down 2 and bringing more people. I would not call it "illegal," but it is dishonest. The extra charges are usually around $15/day per person, so it is not that much of a savings. The best way to save is to look for air/hotel vacation bargains and go during a time of year when they are offered.