A person who enters into a party or other social event without an invitation, or into a theater or other public performance without a ticket.

a person who attends a party without an invitation, or a show without paying admission

someone who gets in (to a party) without an invitation or without paying

a person or people the come to somebody's party unannounced and walk/force their way in, alot of the time if in a large group trash the persons home and steal things

Gatecrasher, originally slang but now in common usage, refers to a person who enters some event without a ticket or invitation. The term gatecrasher, though it sounds like someone battering down a barrier, more typically refers to someone sneaking in, or talking their way past a ticket taker, or some other stealthy technique, rather than a confrontational encounter.