Their families, I guess. They already know about it before the play starts, since they know about it in Act 1 Scene 5 and we don't see anyone telling them.
they find out just after the party he crashed. benvolio tells them
Romeo's servant Balthazar tells him the news from Verona.
Benvolio tells Romeo that Juliet is a Capulet at the party.
Juliet's Nurse tells Romeo that Juliet is a Capulet, while Romeo's friend Benvolio reveals to Juliet that Romeo is a Montague.
It confirms the story that Friar Lawrence tells the families. It tells what has happened over the four days that only Romeo, Juliet, Nurse, and Friar Lawrence knew. About how Romeo and Juliet fell in love and married, etc.
at first she doesn't, but he tells her he is there eventually
Juliet's Nurse tells both Romeo and Juliet who the other's identity was.
When Romeo tells Friar Laurence that he's in love with Juliet, the Friar is initially surprised but then agrees to help them get married in hopes of ending the feud between their families.
Balthasar
The Nurse
Romeo's friend Balthasar tells him
Benvolio(Romeo's Cousin), tells Romeo to forget Rosaline (Romeo's love before he met Juliet) and to go to the Capulet's party to meet other girls