Mr. Heckles, the saviour!
Don't you think that Joey is in the gang because of Mr. Heckles. In the one with flashback, It is shown that the other photographer guy gets chosen to be chandler's roommate but when Mr. Heckles tricks him by saying, "I could be Chandler's roommate" he believed that Mr. Heckles was the roommate. So he left, and Chandler had to ask Joey to be the roommate even though he didn't want him (PS: Monica did want joey to be the roommate 'cause he was hot). When Monica thanked Chandler for picking him, he stated, "Thank that jerk who never showed up". He didn't know that this roommate would become his best friend.