Does the following sentence have an antecedent? If so, where?: Some people came by to offer their help. They were really friendly.
Yes. The antecedent is people.
An antecedent defines the pronoun. If I say, "Sammy has ticks. I need to take him to the vet." You know that "him" in the second sentence refers to "Sammy." In this sentence, you know that "They" in the second sentence refers to "some people."