David Cassidy died last week after suffering from multiple organ failure. Before this, in February, the actor and singer had revealed that he'd been diagnosed with dementia so would no longer be playing concerts.
He starred in The Patridge Family, which produced a number of hit songs. This included I Think I Love You, which reached No. 1 on the Billboard chart in 1970, the year the sitcom debuted.
Cassidy later launched a solo career, recording a dozen studio albums and six compilation albums.
Elton posted a photo on Instagram, saying, ''Here I am introducing him to my friends Tony King and Ringo Starr.''
He added that they were good pals in the '70s who had ''so many great times together,'' and he was sad to hear of the 67-year-old's passing.
Brian Wilson said he and David once started writing a song together.
"He was a very talented and nice person," he recalled.
Larry King said that like his father Jack Cassidy, the former teen heartthrob had ''great talent and a complicated life.''
David, who appeared on shows from The Love Boat to Celebrity Apprentice, was married and divorced three times. Survivors include his daughter Katie Cassidy, who is on The CW's Arrow.