why google when you have Smiley Smile?
(I googled anyway)
https://en.wikipedia.org/wiki/Guess_I%27m_Dumb Glen Campbell – lead vocals, twelve-string acoustic guitar
Additional musicians
The Honeys
Marilyn Wilson
Diane Rovell
Ginger Blake – backing vocals
Arnold Belnick – violins
Hal Blaine – drums, timbales, bell tree
Roy Caton – trumpet
Harry Betts – trombone
Louis Blackburn – trombone
Steve Douglas – tenor saxophone
Jesse Ehrlich – cello
Jim Getzoff – violin
Anne Goodman – cello
Larry Knechtel – electric bass guitar
Leonard Malarski – violin
Jay Migliori – saxophone
Oliver Mitchell – trumpet
Alexander Neiman – viola
Sidney Sharp – violin
Tommy Tedesco – guitar
Darrel Terwilliger – viola
Brian Wilson – backing vocals, acoustic grand piano
Carl Wilson – backing vocals, twelve-string electric guitar
Chuck Britz – engineer
Brian Wilson – producer, mixer