Robert Anton Wilson the "science fiction fan"
Robert Anton Wilson, who more or less by accident is known as a "science fiction writer," also can be described as a "science fiction fan."
Science fiction fandom arose in the 1930s or so as an organized activity by aficionados of the genre. They quickly began putting out fanzines, organizing conventions, forming clubs, corresponding with each other, and so on. Many famous writers started out as fans, including Isaac Asimov.
While Wilson never immersed himself into fandom, he did attend a number of science fiction conventions (I don't know how often or how many). There was an attempt years ago to organize a science fiction convention in San Francisco at which Wilson would have been the guest of honor; it fell apart when the hotel arrangements fell through.
He also, for a time, belonged to The Golden APA. In COINCIDANCE, he refers to it as a "little magazine," but it actually was an amateur press association, or APA. Members of an apa produce a certain number of fanzines -- perhaps 20 or 30 or so -- which are mailed in to a central mailer, who staples them together and then sends them out as a mailing to every member of the group.
Participating in apas was a hardcore activity of science fiction fandom (I myself was in APA-50 and, for awhile, FAPA). The Golden APA, as the name implies, tended to attract libertarian SF fans and Wilson fans.