party crasher
One who attends a party without having been invited. I don't mind party crashers coming to my parties, so long as they behave themselves.
gatecrasher
One who attends an event, such as a concert, sports match, or party, without being invited or paying to get in. I don't mind gatecrashers coming to my parties, so long as they behave themselves. Due to the popularity of the show—and the limited amount of tickets—extra security was hired to keep any gatecrashers out of the concert.