No one "joins" atheism. It is simply a position of belief. It isn't a club, or a religion, or even a 'movement.' If you do not accept the claims of religion and the existence of God, you are an atheist.
The only Con of atheism is that in most parts of the world atheism is still a minority. And, is often the case, being a minority, especially a minority demonized by the majority (consider that most members of religions view atheism as inherently immoral), can cause problems in your life. There are many, many examples where families have ostracized relatives who are openly atheist, that atheists who stand up for their right to be an atheist to are sent death threats, billboards for atheist organizations are vandalized or are sent threats, that atheism is literally illegal in some countries. etc.
The Pro... I feel no compulsion to give my money to millionaires who promise me they will cure me or make me rich through prayer.