어 서다

come to a stop; stand in line

The clearest use here is 멈춰 서다 — stopping and standing still — which three lines have: 몇 걸음 못 가서 또 멈춰선 "stopped in their tracks again after a few steps", 유나 생각에 울컥 멈춰서는 "stopping short, choked up". 서다 adds the standing-still part that plain 멈추다 does not carry. 줄지어서 in another line is lining up in a row, the way a queue forms. Note that a couple of the remaining lines are really 붙어 있다 "be right next to each other", not this pattern at all.