Unexpected Guest

One time, my girlfriend and I had a friend staying with us for the weekend. We spent the night partying and had lots of fun. When we arrived back home my girlfriend and I immediately started making out heavy in our bedroom. I started going down on her and her moans became louder. Suddenly we noticed that our friend was standing in the door, watching us while touching herself. My girlfriend and I exchanged looks and invited her to join us.. this is when the night became super hot...