It's actually a phrase which, in this context, actually means to attract people towards her melodious voice. :-)

For the literal meaning, imagine that she is standing at the door of the venue. Whenever someone walks by, she reaches out. She grabs them. She pulls them in through the door. She pulls in one concertgoer, then another, and another, until she has pulled in a huge crowd. It is just like a fisherman pulling in fish. Of course she doesn't really do that. Figuratively: she is wonderful. She has thousands of enthusiastic fans everywhere. When she announces that she is having a concert, she "pulls in" a huge crowd. Yes, the crowd "congregates." But the reason is they are behaving as if there were an attractive force that was pulling at them. You can also say: "She attracts a huge crowd whenever she performs," or "She draws a huge crowd whenever she performs."