This happened with me once. There was me, another girl, and a guy left after a party at the guy's house. We were all naked from playing strip games at the end of the night. The guy started cleaning up and I went to the bathroom. When I got back the other girl was watching porn on the TV. I sat down next to her before I realized she playing with herself. I started fingering myself while we both commented on the porn. That got the guy's attention, he sat next to me and started jerking off and commenting on the porn with us.