gatecrasher

from WordNet (r) 3.0 (2006)

gatecrasher
    n 1: someone who gets in (to a party) without an invitation or
         without paying [syn: {gatecrasher}, {crasher}, {unwelcome
         guest}]

from The Collaborative International Dictionary of English v.0.48

gatecrasher \gate"crash`er\ v. t.
   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.
   [PJC]