While the song is about the formation of the Mamas and the Papas, it mentions other artists who were getting their starts at the same time:
Sebastian is John Sebastian, who, with Zal Yanovsky (also mentioned in the song), formed the Lovin' Spoonful.
"McGuinn and McGuire, couldn't get no higher" - Jim "Roger" McGuinn
was the lead singer for The Byrds, who were starting to take flight at the time and who had two #1 songs in 1965 - "Mr. Tambourine Man" and "Turn, Turn, Turn." Barry McGuire had a #1 in 1965 with the anti-war song "Eve Of Destruction
." In other words, at #1 they "Couldn't get no higher."