When Fritzi reintroduces herself to Jill at the start of the movie and Jill fails to remember her, Fritzi reminds Jill that the previous summer, they had been in the play "'night, Mother" together. The joke is that "'night, Mother" only has two actors in it, and is an extremely intense, wrenching, emotional experience (it is about an adult daughter preparing her elderly mother for the fact that the daughter is going to commit suicide), so there is no way that Jill could have forgotten having already met Fritzi without Jill being incredibly self-absorbed.
The song Ellen listens to from her "Broadway ballads" mix CD, is the song "Imagining You" which is from the musical "Birds of Paradise". Writer and director, Todd Graff, starred in "Birds of Paradise" when it first premiered Off-Broadway.
In joke: Bert Hanley's character, a washed-up songwriter turned director at summer camp for theatrical kids, has the same name as the unseen business manager mentioned in 1962's Whatever Happened to Baby Jane, involving Bette Davis as a former child star.