I haven't read the article yet, but the answer is simple. Just be a fan. Start watching the games if you can anyway you can or keep track of the games by checking the sports section of your local paper, or now-days the sports websites for the scores. Simple. Do that for all the teams you like and you are then a fan. Simple.
Or wait, are we supposed to fill out some complicated membership form and wait 3 seasons for it to be approved by the team's management?