Niall Horan

Ariana Grande and Niall Horan hung out way past their bedtime Monday night -- but don't get your panties in a bunch just yet 1Ders. Our Ariana sources say the One Direction singer was invited to her concert in London Monday night. After the performance, she and Niall hung out backstage, but it was…

Harry Styles paid tribute to 1D, as in ... DEEZ NUTS ... in Las Vegas. One Direction won the Billboard Music Award for Top Duo/Group, and Harry celebrated by slyly grabbing a hold -- playfully and gently, we hope -- of bandmate Niall Horan's balls. Watch ... Niall plays it off pretty good. Not…