In Act 2, scene 2, the Players, a group of traveling actors, arrive. Typically, acting companies like this one traveled the country to provide entertainment and often share news. Hamlet knows these actors, and he's familiar with the plays they present. Delighted by their arrival because of the diversion they will provide from his gloomy state of mind, he also decides to employ them in a trap he will set for Claudius.
Hamlet will compose a few lines of his own for the Players to insert in a play known as "The Murder of Gonzago." Hamlet's addition, which will replicate the murder of Old Hamlet, is named "The Mousetrap." He will use this play "to catch the conscience of the king"("2.2.560), as he says at the conclusion of Act 2.
In Act 5, scene 2, another group arrives when the Norwegians led by young Fortinbras arrive, apparently to attack Elsinore, only to find the kingdom virtually bereft of leadership.