Rihanna stripped down in the wee hours Monday morning after arriving with 20 pals at Pink Elephant. The “Diamonds” singer showed up at 2 a.m. in a black jacket, but then, “She jumped on a banquette as music thumped, and started peeling off her jacket . . . then her shirt . . . to ultimately reveal a red tube top.” Among those getting an eyeful of the exhibition was “Beautiful Girls” singer Sean Kingston. The club’s regular crowd then exited at 4 a.m., but Elephant owner David Sarner kept the music blaring for RiRi and her posse, so they could dance for another hour. “They didn’t serve any more alcohol, it was just so Rihanna could keep on dancing,” a source said. The singer then left a $500 tip on her more than $2,000 tab. Rihanna was in a particularly revealing mood: She’d earlier posted a pic of herself wearing an outfit from her River Island fashion line, with nothing underneath and showing a healthy dose of side boob. She also sassily tweeted a message seemingly aimed at her ex, Chris Brown, exclaiming, “Got a thang for a King, but chu ain’t a King!!”