From the "12 Step and 12 Tradition" book:

"The only requirement for AA membership is a desire to stop drinking"

  "... AA is really saying to every serious drinker, 'You are an AA member if you say so. You can declare yourself in; nobody can keep you out. No matter who you are, no matter how low you've gone, no matter how grave your emotional complications - even your crimes - we still can't deny you AA. We don't want to keep you out. We aren't a bit afraid you'll harm us, never mind how twisted and violent you may be. We just want to be sure that you get the same great chance for sobriety that we've had. So you're an AA member the minute you declare yourself...

"To establish this principle of membership took years of harrowing experience. In our early time, nothing seemed so fragile, so easily breakable as an AA group. Hardly an alcoholic we approached paid any attention; most of those who did join us were like flickering candles in a windstorm. Time after time, their uncertain flames blew out and couldn't be relighted. Our unspoken, constant thought was 'Which of us may be the next?'...

  "' At one time... every AA group had many membership rules. Everybody was scared witless that something or someone would capsize the boat and dump us all back into the drink. Our Foundation office asked each group to send in its list of 'protective' regulations. The total list was a mile long. If all those rules had been in effect everywhere, nobody could have possibly joined AA at all, so great was the sum of our anxiety and fear...

  "We were resolved to admit nobody to AA but taht hypothetical class of people we termed 'pure alcoholics'. Except for their guzzling, and the unfortunate results thereof, they could have no other complications. So beggars, tramps, asylum inmates, prisoners, queers, plain crackpots, and fallen women were definitely out. Yes, sir, we'd cater only to pure and respectable alcoholics! Any others would surely destroy us. Besides, if we took in those odd ones, what would decent people say about us? We built a fine-mesh fence right around AA...

  "How could we then guess that all those fears were to prove groundless? How could we know that thousands of these sometimes frightening people were to make astonishing recoveries and become our greatest workers and intimate friends?... Could we then foresee that troublesome people were to become our principal teachers of patience and tolerance?...

  "... At last experience taught us that to take away any alcoholic's full chance was sometimes to pronounce his death sentence, and often to condemn him to endless misery. Who dared to be judge, jury, and executioner of his own sick brother?

  "As group after group saw these possibilities, they finallly abandoned all membership regulations. ... The hand of Providence early gave us a sign that any alcoholic is a member of our Society when he says so."