How about one with Serena and Grace?
About my suggestion about Serena and Grace, Grace comes home from shopping and hears moaning from upstairs. She walks up the stairs and hears Serena say, " Faster Ash." She then opens the door to Serena's bedroom and is shocked at seeing Ash and Serena in the bed fucking doggy style. Then she notices how far back Ash moves without pulling out of Serena and is turned on by Ash's 12" cock moving in and out of Serena's cunt. She waited until they came together before announcing her presence and asked if she could join in on the fun that they were having. You should be able to take it from there.