If you join an organization, you become a member of it or start work as an employee of it.
参加 (组织) • He joined the Army five years ago. 他5年前参了军。
To join two things means to attach or fasten them together.
连接; 接合 • The opened link is used to join the two ends of the chain. 这个开口环是用来连接链条两头的。