Jon Hamm and Edward Norton were also among the guests who took in Bernard Pomerance's drama at the Booth Theatre.
The event honored director Michael Mayer and publicist Rick Miramontez.
The new musical, inspired by Jonathan Lethem's novel, plays the Public Theater.
The new play directed by Garry Marshall stars Larry Pine and Vincent Kartheiser.
The organization will celebrate the Tony winner at its annual gala.
The new drama at The Public Theater is written by Scott Z. Burns.
Westfeldt was joined by her mother and boyfriend, Jon Hamm, for the Boston theater's opening-night festivities.
Why the Tony-nominated actress returned to the stage and how she may do a play with her partner, Jon Hamm…once he can speak again.
The big top-bred brother and sister have soared to Broadway and beyond.