The full quotation is, "You're either on the bus, or off the bus." Purportedly said by Ken Kesey at a moment when one of the riders was threatening to leave Furthur during the cross country journey of the Merry Pranksters. This is documented in the book the Electric Kool-Aid Acid Test by Tom Wolfe. My reading of this has less to do with whether you "get it" or not. It has more to do with, "If you're leaving do it and let us get on with the program. Make a choice."

"On The Bus" is also the title of a 1990 book that has an introduction by Ken Babbs published by Thunder's Mouth Press. The full title is "On The Bus: The Complete Guide to the Legendary Trip of Ken Kesey and the Merry Pranksters and the Birth of the Counterculture."

Source: Last Updated 06.20.02