Crystal and Peter are the subjects of this sentence. The multiple subjects are called compound subjects.
Subjects and verbs pair off, but sometimes you get two (or more) for the price of one. You can have two subjects (or more) and one verb.
Here you notice one action (bought) and two people (Crystal, Peter) doing the action. So the verb went has two subjects.
Every complete sentence contains two parts: a subject and a predicate. The subject is what (or whom) the sentence is about, while the predicate tells something about the subject.
The subject of a sentence is the person, place, thing, or idea that is doing or being something. You can find the subject of a sentence if you can find the verb.
One way to think about the subject is to say that the subject is the who or what part of the subject-verb pair. The subject-verb pair is the main idea of the sentence, stripped to essentials. See examples on Dummies.com and on Grammar.ccc.commnet.edu.