>Someone is knocking at the door, can you let them in?
That is not a good example since there is presumably only 1 person knocking ("someone"), but there could be multiple people ("them") at the door waiting to come in.
Since you don't know the amount of people waiting to come in, using them would be appropriate.
That is a better example for sure. I still think there are issues since it could be somebody other than one of your parents knocking (including multiple people)
I think the better example would be when you know there is only one person. Person 1: "I went to the movies with a friend". Person 2: "Did they like the movie?"
This still has the issue about talking about somebody who is not present. Many people are advocating for using they/them when the person is right in front of you.
That is not a good example since there is presumably only 1 person knocking ("someone"), but there could be multiple people ("them") at the door waiting to come in.
Since you don't know the amount of people waiting to come in, using them would be appropriate.