Someone who gets in (to a party) without an invitation or without paying; SYN. crasher, unwelcome guest. gate-crasher

One that interlopes: as an illegal or unlicensed trader; one that intrudes in a place or sphere of activity

Someone who intrudes on the privacy or property of another without permission; SYN. interloper, trespasser.

Someone who enters by force in order to conquer; SYN. encroacher.