OK! Exclusive: Ellen & Jen Hit Hollywood Baby Bash

Jennifer Aniston, Dustin Hoffman, Ben Stiller, Ellen DeGeneres, Jack Black, Tobey Maguire, Kate Beckinsale, Jonah Hill... That's a list of stars that you'd expect to see at an exclusive blockbuster movie premiere or major awards show.

But on Sunday afternoon, these A-listers were making a very different kind of scene as they all attended the first birthday bash for Sacha Baron-Cohen and Isla Fisher's baby daughter, Olive.

Jen, who made birthday headlines earlier this week when she celebrated on-off-on beau John Mayer's 31st, arrived at Isla's Hollywood Hills home hiding on the floor of the backseat of BFF Courteney Cox and David Arquette's car.

Proud mom Isla greeted each of the celebrity guests at the front door as they arrived for the party. Oscar-winner Dustin Hoffman arrived just as Jack Black and his family were leaving. The Rain Man star made a big scene of his disappointment, pretending to smash his gift on the hood of Jack's car and then playing peekaboo with onlooking photographers from behind the oversized package.

Getting into the party spirit, David Arquette acted as both parking valet and security guard for the cowgirl-themed bash. He even wore a t-shirt with the word "insecurity" emblazoned on the back, as he did his best to park the party guest's cars and keep the paparazzi at bay.

Though witnesses tell OK! he could have done a bettter job with Ellen's new Mercedes, which he managed to get wedged between two other cars.

When OK! spotted Ellen and wife Portia de Rossi leaving the party, we asked if they had any valet-parking tips for David. "Stay in school!" Joked Ellen.