The UESD takes the stage, in order of Why Is Any Of This Happening.

Inès Serrano: "Put down your glasses! Do not toast this union! Do not marry this woman! Her name isn't even Sabrina! She doesn't even go to this school!"
Valet: "As a crazy person myself, I am proud to present you a quick précis on what I see as Serena/Sabrina's major faults. I'm talking history of mental imbalance, drug use, promiscuity, rap sheet, snuff film..."
Joseph Garcin: "She's my sister, sometimes, and I am actually pretty worried about her. Also, I will fuck all of you. Just putting that out there."
Estelle Rigault: "Guys, wait. This just in, we are at a gay wedding. See those dudes up there dressed like Chuck? I spotted them, because I am a journalist. I think we effed up again. Repeat, this is a gay wedding."

Hell: Is other people. Specifically these ones.

UPSTAIRS

Sabrina: "...Because having known me for three months is enough to make me their Maid of Honor, that's why. The gays love me."
UESD: "We found drugs!"
Sabrina: "Uh, I'm sure you did. Barry Watson is a holistic health manufacturer."
Chuck: "Wait, you're Steven Spence. I saw your TED talk!"

Nope.

Blair: "But Serena, you disappeared! That's still in play!"
Sabrina: "Uh, I did what you said. You kicked me out, I went. And Dan and Nate didn't want anything to do with me..."
Nate: "Did I really say that? Meh. Who knows."
Sabrina: "So I picked myself up off that traincar floor where I was dead, wiped a fair amount of vomit off whatever I call clothes, came up with money out of nowhere whatsoever or else you would have been able to track me, and created a new identity for myself. Sorry, Steven Spence."
Steven Spence: "It's okay. I don't really love you because of your name or where you matriculate, or even because you are from Wisconsin. I think we all know why I love you. And they're not going anywhere."
Sabrina: "Aw, you did notice! You are a hell of a guy. Unlike these jokers. Usually when I act out by disappearing, you figure it out within weeks."