If you intrude on people without their permission, you are an interloper. An interloper crashes parties and laughs at "No Trespassing" signs.
Interloper was likely formed by combining inter-, ("between") with -loper, (from landloper, meaning "vagabond" or "adventurer"). An interloper, then, is someone who has ventured or traveled where he was not welcome. The noun interloper may also refer to something other than a person: "The new chain superstore built at the edge of town and was an interloper among the various mom-and-pop grocery stores in the area."