This is the 6874th most frequent Korean word.
입구에
"At the entrance."
입구에 is used to specify the location where people are gathered, indicating a place or position.
입구에 사람이 많이 모여 있었어요.
There were many people gathered at the entrance.
입구에 here describes the place of arrival upon which the action of checking in takes place.
우리는 입구에 도착하자마자 한 명씩 체크인을 했습니다.
As soon as we arrived at the entrance, we checked in one by one.
입구에 describes the location where the sign is placed, emphasizing visibility at that location.