The band's origin is a bit more complicated than that having its roots in a couple of Chad Allen-led bands. One of the single releases of Chad Allen and the Expressions was credited to "Guess Who?" as a publicity stunt. This was pre-Burton Cummings. He joined the band after that and just before Chad Allen left but the name stuck.
Guess Who was very talented. IMO Burton Cummings and Mike Smith of the Dave Clark Five are the two most underrated singers of the pop era.