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.
Friar Lawrence relates the story of Romeo and Juliet to the crowd that's gathered outside the Capulet tomb via the Prince, who's demanded knowledge of the situation, justifying his part in the tragedy and relaying the fateful twists that have lead to this scene.
Acknowledging that he's a "holy man," true to his word, Paris asks Balthasar, Romeo's page, to corroborate and fill in the gaps. Asking the same of Paris's page, the Prince determines that the stories and the Friar's letter concludes the tale of woe.
The story is first told to the Prince, who has demanded the truth of the events, by Friar Laurence. He relates to the Prince the entire story of Romeo and Juliet.
The Prince then turns to question Balthasar who relates how he is the one that told Romeo that Juliet was dead, and how he and Romeo came to be at the tomb and of the letter that Romeo gave him to give to his father.
Paris' page then tells the Prince that he and Paris came to the tomb for Paris to leave flowers for Juliet, how Romeo came, and the fatal fight between Paris and Romeo.
The Prince then reads the letter that Romeo left for Balthasar to give to Romeo's father.
After hearing all accounts, the Prince declares that the Friar has told the truth about what has happened and what has led to the deaths of not only Romeo and Juliet that night, but his kinsman Paris as well.
it's friar lawrence who tells the story.