Namely the reason Gold Joinee Brake picked up the book Join Me - in order to research the whole idea of developing a cult personality. In the end he considered it more worthwhile to join an existing cult based on the good deeds inspired by Danny Wallace, than to create a completely fabricated cult based around a completely fabricated person.
Note that the the face of Doctor Celery Jones belongs to an actual person called Chris Jones, identical to the name of Danny's first Joinee. It's this weird little coincidence that helped lead Simon towards joining. But then the world of Join Me seems full of little coincidences.