buttinsky (n.)

• admonisher
• adviser
• confidant
• consultant
• counsel
• counselor
• crasher
• encroacher
• expert
• guide
• infiltrator
• instructor
• interloper
• intruder
• invader
• kibitzer
• meddler
• mentor
• monitor
• nestor
• nose
• pragmatist
• quidnunc
• rubberneck
• snoop
• teacher
• trespasser