Complex sentence: Meanwhile, a group of "popular girls" who did not receive pins are not let in, so they stomp off, while the girls with pins are permitted to enter the concert where Emblem3 performs on stage, singing and dancing during a performance of Chloe.
Split into simpler sentences: Meanwhile, a group of so-called popular girls who did not receive pins are not let in so they stomp off, while the girls with pins are permitted to enter the concert. Then, Emblem3 performs on stage, singing and dancing during a performance of Chloe.