Mike Love is the one in the cap. By this time, Brian seldom performed with the band, although not never.
That is Bruce Johnston on the left (stage right). He joined in 1966 to replace Brian on stage, after the latter went into the studio full time, and after Glen Campbell left the touring band. He first recorded vocals with them on Pet Sounds. (Campbell did too, on guitar, as he was one of the Wrecking Crew).
Johnston penned, I Write the Songs, sung by Barry Manilow. But the song is really about Brian Wilson, genius composer. Johnston also wrote the backing vocals for Pink Floyd’s The Wall, as I recall.