One thing that becomes noticeable when you read Batman is that he (both in his costumed identity and as Bruce Wayne) was an inveterate joiner. I am going to try to chronicle all the various clubs that he participated in; it's going to be quite a list.
Batman #40 showed our hero becoming the 13th member of the 13 Club, a group dedicated to wiping out superstition:
Bruce belonged to the the $1,000,000 Star Club, which, if I recall correctly, was made up of people who had been insured for $1 million or more.
In World's Finest #50, it was revealed that Batman was not just a member of the Bullet-Hole Club, he was its president. It was a dubious honor, considering the requirements:
Batman joined the Danger Club in Batman #76:
He appeared numerous times in the Justice League of America (and once in the Justice Society of America).
He was a charter member of the Mystery Analysts of Gotham City:
The Dynamic Duo were made honorary members of the Safari Club in Batman #111:
We learned that Batman belonged to the Voyagers Club in Detective #147:
It should come as no surprise to learn that millionaire playboy Bruce Wayne belonged to the Yacht Club (Batman #49):
Bruce was also a member of the Hobby Horse club, for collectors:
Any others you can think of?