I agree with ms-t and the others. Even though the "Chorus" who begins and ends the tale is not directly identified, that person must be Prince Escalus, who demands the others recount the tale. He learns the truth primarily from Friar Lawrence, who is intimately involved in the details of the lovers' plans. Let me tell you why I believe the prince begins and ends the play.
Shakespearean tragedies always follow a formula.
Act I is the exposition, where the characters and conflict are introduced. (We learn of the feud between the families, and Romeo and Juliet meet and fall in love.)
Act II is filled with rising action and complications. (Romeo and Juliet declare their love and marry, while Juliet's parents are planning her marriage to Paris.)
Act III is always the climax, the point where the main conflict comes to a head, usually in a less that positive way, and the tragic hero's fate is sealed. (Romeo kills Tybalt as a result of the initial feud and is banished.)
Acts IV and V show falling action-the things that happen as a direct result of the actions that happen in Act III. (Romeo is banished. Juliet's parents plans force her to fake her death. Romeo receives erroneous information about Juliet.)
Act V always contains a catastrophe. The tragic hero (along with others) always dies and some lesson is learned by that death.
Also, invariably, in every Shakespearean tragedy, the last person to speak is the person who is left in charge of the situation--or the new or current ruler of the land. In Romeo and Juliet, that person must be the prince.