Who sent an invite to Kellan Lutz and John Mayer telling them near-reflected dark suits were dress code of the opening the Cosmopolitan's New Year's Eve party in Vegas last night?

If YOU were invited to the opening of the biggest hotel and casino on the strip -- a concert with Jay-Z and Coldplay and half of Hollywood flown in to celebrate would YOU were either of these suits???

At least John Mayer redeemed himself, jumping on stage for an impromptu performance including a crazy guitar accompaniment for Jay-Z's 99 Problems.

Other highlights of the night included Coldplay's super rendition of Yellow and a duet with Jay-Z and surprise performer Kanye West. Beyonce jumped on stage to sing Forever Young with her hubby. And Jay-Z, Kanye and Chris Martin sang Homecoming as a Trio.

The $3.9 billion hotel spared no expense on opening weekend -- to other resorts on the Strip: Beware! There's a new player in town!

Check out Beyonce and Jay-Z's Forever Young ...