interloper
/ˈɪntələupə(r); ˌɪntɚ`lopɚ/ n
person who is present in a place where he does not belong, interferes in sth which is not his affair, etc; intruder 闯入者; 干涉他人事务者
Security guards were stationed at the door to deal with any interlopers. 门口已设警卫人员以阻止闯入的人.