To join Al-Qaeda, as to do virtually everything else, you have to fill out an application and be accepted.
Michelle Malkin posted this in January 2006, but ABC News has just noticed, and in light of Padilla’s trial it may be worth another look.
“Key Evidence in Padilla Case: An Al Qaeda Job Application” — story from ABC News.
And here is a pdf of the full application in Arabic, with English translation.