1) Burglar 2) Butter-in 3) Entrant 4) Gate crasher 5) Impostor 6) Intruder 7) Persona non grata 8) Prowler 9) Trespasser 10) Unwelcome person 1) Crasher 2) Encroacher 3) Gatecrasher 4) Intruder 5) Invader 6) Penetrator 7) Prowler 8) Pusher 9) Stranger 10) Thruster 11) Trespasser 
• (n.) One who interlopes; one who interlopes; one who unlawfully intrudes upon a property, a station, or an office; one who interferes wrongfully or officiously.
In'ter·lo`per noun [ Prefix inter- + Dutch looper a runner, from loopen to run; akin to English leap . See Leap , and confer Elope .] One who interlopes; one who unlawfully intrudes upon a property, a station, or an office; one who interferes wrongfully or of...
[Wuthering Heights] an 
One that trespasses on a trade monopoly, as by conducting unauthorized trade in an area designated to a chartered company; a ship used in unauthorized trade.
[SAT terms] someone who intrudes on the privacy or property of another