Clearly the Janitor would join the crew by offering his janitorial services to the Doctor, who would forever fail to see him as anything but polite and professional, especially once he (accidentally?) fixes the chameleon circuit during his day to day janitoring.

But in all seriousness, YES THE JANITOR NEEDS TO JOIN THE TARDIS CREW. Consider the Janitor vs. the Daleks; they'd be all "Exterminate" and he'd be unfazed and just carry on mopping, then he'd be like "Oh yeah, did you drop this?" and pull out some small but vital Dalek component without which they instantly explode, and right then the Dalek explodes.