Season 7: Episode 153: The One With Ross's Library Book
Ross takes Chandler to the university library to find his doctoral dissertation. They encounter a couple of students making out in front of his book. Ross soon finds out that this is a popular spot for couples on campus. Ross decides to guard the book and keep any "extracurricular activities" from taking place. Joey spends the night with a woman named Erin who he decides he doesn't want to see again. He asks Rachel to break it to Erin for him, but when she gets to know her, Rachel grows attached. She and Phoebe convince Joey to date Erin and he ends up falling for her. Unfortunately for Joey, Erin doesn't feel the same way about him. Monica runs into Janice, Chandler's ex-girlfriend, and invited her to the wedding. Chandler warns Monica that Janice attending their wedding is a huge mistake and they must un-invite her