You join an organization-- that is, become a member. Joining generally suggests becoming an intrinsic part of a group. You attend meetings, conferences, classes, etc, where your attendance is not necessarily so important to the function-- you are usually there just to sit and listen.
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You attend meetingsI would not recommend 'join' for meetings of any size. I know that you will discover exceptions to any suggestion I make, but it has been my experience that my Japanese ESL students invariably use 'join' when they mean 'attend' or simply 'go to'.