gatecrasher       
noun [ C ]  informal uk/ˈɡeɪt.kræʃ.ər/ us/ˈɡeɪt.kræʃ.ɚ/
 (聚会、活动等)不请自到的人,不速之客
someone who goes to a party or other event when they have not been invited
  The party was ruined by a couple of rowdy gatecrashers. 聚会被几个吵吵闹闹的不速之客破坏了。 
 There were at least forty people at the party, some of them gatecrashers. 派对上至少有四十个人,其中一些是不请自来的。 
 See: gatecrash
 - More examples
 - He tried to bring dozens of gatecrashers to the party with him.
 - Gatecrashers were evicted from the auditorium to jeers and whistles.
 - The party spun out of control because of a gatecrasher.