I had thought it was a solo act with sweeteners added in studio. I was wrong. They are rather the end of an era for 1969, though, aren't they? Doo-wop hanging on. Still, I sang along in the car today, which is why I posted it here. It is a musical trick to inject excitement into a song by modulating up a half step*. Note how many times the use that.
* And in American Musical theater, if you also slow down, everyone expects that you are going immediately into a kick line, and will begin applauding, Pavlov-style.