The girls had already agreed at the airport to go to a party with the guy who chatted them up (Peter?). They were waiting for a car to arrive at 9pm with him and a couple of his mates - they'd get in and start drinking and head off who-knows-where. But instead of doing it that way, why did they bad guys go into the posh apartment block and drag them out screaming?
shareGood point
shareProbably because Amanda revealed that "We have the whole place to ourselves" so they knew they girls were alone, so decided to strike then.
Plus an apartment block is a lot easier to access that a hotel where I guess more travelling girls would stay. Most places you need a key for the elevator as well as the room. So the kidnappers would probably have to wait outside and pick the girls up to take them to the "party"
That's probably what happened to the girl who Bryan saved (the one who had Kim's jacket) When we saw her flashback she was in a car with a guy looking glammed up and then he said "I'm sorry" and she was dragged into a house.