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Jay-Z Plays Host to Practically Every Celeb Ever for New Year's

If you haven't heard already, the grand opening of the Cosmopolitan Hotel in Las Vegas was the place to be for New Year's Eve.
/ Source: E!online

If you haven't heard already, the grand opening of the Cosmopolitan Hotel in Las Vegas was the place to be for New Year's Eve.

Jay-Z and Coldplay performed for a who's who of celebs at midnight on Friday--with surprise appearances by Kanye West, Beyonc and John Mayer--and the party hardly stopped there.

So who was Blake Lively's party partner? Were Cameron Diaz and Alex Rodriguez really all lovey dovey? And is Mark Salling actually dating Rumor Willis?

Read on to find out who was not kissing who at midnight...

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Newly single Blake looked fabulous as usual, showing off her perfectly toned arms in a black sparkly minidress.

While Penn Badgley was looking a little toasted (and hardly hurting for female company) as he partied it up in Miami, all eyes were on Lively as she danced the night away at Marquee nightclub following the concert.

"Guys were drooling all over her," says a source inside the bash. "Despite some brave ones who tried their best to hit on her, instead she hung tight with a few girlfriends she was with, singing along to Rihanna, who gave an impromptu performance at the afterparty."

Blake's bud happened to be Florence Welch from Florence + Machine, as we're told the two were inseparable all weekend.

Hmmm, maybe her girls-only weekend had a little something to do with Ryan Gosling?

One couple fully on were Diaz and A-Rod. The two were very affectionate as they watched Florence + Machine perform at the grand opening of Marquee Thursday night, holding hands as they mingled with the likes of Jay Z and Beyonc.

"They looked really happy together, dancing and even stealing a few kisses," dishes an onlooker.

And their love fest didn't stop there. At the afterparty at Marquee on Friday night, Cameron and Alex were seen making out nonstop and cuddling!

However one non-couple we can definitely tell you about is Mark Salling and Rumor Willis.

Rumor, who dated Salling's bud Chord Overstreet, was rumored to be romancing the dark-haired Glee hunk, but this is way false.

"They are definitely not together," 'fesses a bud close to Salling. "And never have been."

Instead, Mark, looking way yum (he was spotted shopping at Guess before the concert), hung tight during the two-day bash with his guy friends, including actors Ryan Rottman and Kellan Lutz.

Sounds like everyone was too busy having a blast to cause trouble in Vegas! How unusual.

Just a few other celebs in attendance at the insane event:

Gwyneth Paltrow, Jennifer Lopez and Marc Anthony, Mark Wahlberg, Jeremy Piven, Kirsten Dunst, Justin Bartha and Ashley Olsen and Jessica Szohr.

Gwyneth, by the way, wasn't her usual aloof self--jeez, is she gunning for the Oscar nom, or what? She was dancing and singing along at a center table during her husband's performance before making a quick appearance at the afterparty. Their kids were in Vegas too, after all.

PHOTOS: Party Pics: Vegas