The phrase "to join the association" means to become a member or join a group of people who have come together for a common purpose or interest. Full definition
Joining an association of small business professionals, finding out that people had already experienced those issues and found solutions, was one of the most exciting «finds» of my business life. (topmargin.com)
Join an association in whatever field interests you and become an active member. (candocareer.com)
They were led to believe that, by joining the association, they would be able to obtain virtual accounts from banks after the government - mandated real - name system went into effect. (cryptotimes.news)