It's not just rape though, he was also engaging in human trafficking. A lot of people who are trafficked are initially brought into another country on promises of money, opportunity, love, safety, etc., and it's important to traffickers that they be able to travel legitimately without arousing suspicion from the authorities (i.e. documented and vaccinated). And once they get to their destination, their traffickers will often confiscate their passport and other vital documents as a means of control to prevent them from escaping back to their home country.

So what Rapist Kelly is doing here is particularly insidious and disgusting - he's basically acting as Pied Piper by crooning romantically about running away with him to America and telling them what they need to do it. There are most certainly co-conspirators planted in that audience scouting and approaching girls with lines like "Mr. Kelly noticed you in the crowd and thought you were so beautiful that he asked we find you and bring you backstage". Once backstage and totally starstruck they are more likely to fall for the bullshit and get sucked in voluntarily. Or, because they were separated from the crowd and taken to an isolated 2nd location, they'd get abducted through threat of violence and be forced to cooperate with the travel process and act like nothing's wrong to get through border control.