A pronoun replaces a noun in a sentence. The pronoun they takes the place of two or more subject names in a sentence.
Jim and Joanne are traveling to Europe, and Jim and Joanne will visit Berlin.
Jim and Joanne are traveling to Europe, and they will visit Berlin.
The pronoun them takes the place of two or more names in the objective case of a sentence.
I bought a gift for Jim and Joanne, and I gave it to Jim and Joanne.
I bought a gift for Jim and Joanne, and I gave it to them.
The pronoun they replaces the names of two or more people who are doing something. The pronoun them replaces the names of two or more people when action is happening to the people. They do it, but it happens to them.
It’s your turn. Decide if you should use they or them in this sentence. Mom and Dad have an anniversary, and we will surprise _____ with a party. That wasn’t hard, was it?