This girl is no amateur; she's a prostitute who is apparently working the party. Turn up the sound as loud as you can and listen carefully. At the 4:05 mark she very distinctly asks the dude, "You want to do this for free?" to which he replies "Yeah." Watch her mouth at the same time and you'll very clearly see her mouth the word "free" which matches the sound.
ive seen this video from the point of the bedroom a million times and ive always gotten so wet from it...first time ive seen the whole thing...soooo hot!!!