'NSYNC: Timberlake and Crew Celebrate JC's 40th Birthday

There was an 'NSYNC reunion Monday night, and Justin Timberlake performed with the group -- sort of.

Justin, Joey Fatone, Lance Bass, and Chris Kirkpatrick sang "Happy Birthday" to JC, who turned 40.

Timberlake was surly as he left The Nice Guy, telling the paparazzi to "shut the f*** up." Joey, on the other hand, was friendly and hilarious ... and clearly somewhat intoxicated.