(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.

Dutch uncle Paul Polonius admonisher adviser backseat driver butt-in confidant consultant counsel counselor crasher encroacher expert gate-crasher guide infiltrator instructor interloper intermeddler intruder invader kibitzer meddler mentor monitor nestor nose orienter pragmatist preceptist prier quidnunc rubberneck snoop teacher trespasser unwelcome guest