During the scene at Elysium where everyone is sitting around the 'truth circle' and George is given a bowl of Ayahuasca tea to drink, he smells it and passes it to Linda saying that the tea "smells like Willie Nelson's braids". Willie Nelson was the name of Paul Rudd's character's pet dog in the movie Our Idiot Brother (2011) which, incidentally besides Paul Rudd also starred Kathryn Hahn.
When George is standing in line getting coffee at the beginning of the film, the sign behind him reads "New York Office Building (actually in Atlanta)", a site gag obviously referring to the fact that the film was wholly shot in and around Atlanta.
The trailer has a clip of Paul Rudd walking into his room to find Jennifer Aniston lying on the bed with two other women. Rudd asks "What happened?" to which Aniston replies "You just missed it. It was so beautiful." This clip is not in the movie.
At the start of the guitar scene where George and Seth play "Two Princes" you can hear some of the others singing "Love Take Me Down to the Streets" a song written and recorded for David Wain's previous film Role Models (2008). The song is meant to sound like the Paul McCartney band Wings, and is often believed to be a Wings song as a running gag throughout Role Models.