Liza Minnelli played unofficial emcee Monday night at Jay-Z’s show-stopping Carnegie Hall benefit. After walking the red carpet in a cleavage-baring black gown, diamond necklace and full-length fur, the “Cabaret” diva was spotted walking to the concert hall’s stage before Jay began his show, waving to the crowd and calling out, “Hello, New York!” spies tell us. Minnelli was also photographed backstage with Jay-Z (left), impeccably dressed in a white dinner jacket and sunglasses. Nas and Alicia Keys also guest-starred during the landmark concert, the first solo rap performance at Carnegie Hall. Jay unexpectedly thrilled some fans towards the finale by disappearing backstage, then reappearing up in their balcony box. “There were these two banker-looking dudes who Jay just kept pointing to all night, feeling their energy,” said a source close to the rapper. “So he just went up there. They were freaking out.” After performing a medley of hits while standing on a seat, Jay attempted to climb outside the box to the next level of the hall. But the feat for the proud new dad proved too perilous and he headed back down to the stage for a safer sign-off. The rapper later joined wife Beyoncé and well-wishers at his 40/40 Club after-party.