Backstreet Boys in concert
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Viall was joined on stage by his bachelorettes, who shimmied behind him as back-up dancers.
“Nick looked like he was having a blast, ” a witness tells ET. “He kept glancing over to AJ for guidance, but when it came to the chorus he killed it!”
The GRAMMY-nominated group slowed things down with an acapella version of “I Want It That Way, ” while Viall and one lucky lady took center stage for a slow dance.
Singing along, Viall and the brunette cozied up for an intimate dance, which ended in a steamy make-out session.
“The other contestants were in the background dancing along, but some appeared to be bummed it wasn't them dancing with Nick, " the witness added.
The boy band also hyped up the crowd by bringing former Bachelor contestants, Ashley Iaconetti and Becca Tilley, up to the stage and serenading them with fan favorite, “Get Another Boyfriend, ” from their 1999 album Millennium.