Remember that "Friends" episode? The one where Jennifer Aniston turns 51 and parties with some of her closest friends?? Well, allow us to jog your memory. The actress celebrated the big 5-1 Tuesday night at the Sunset Tower Hotel in West Hollywood, where she made a sneaky quick entrance ... but her pals were slow to trickle out. Just a small handful of those in attendance ... her BFF and former "Friends" costar, Courteney Cox, jewelry designer Jennifer Meyer, Kevin Nealon and Chace Crawford.