“加入工会”常见的英文表达是 “join a trade union” 或 “become a member of a trade union”。
“join” 强调加入某个组织或团体的动作；“trade union” 就是“工会”的意思。
“become a member of” 意思也是“成为……的一员” ，同样可以表达加入工会这一行为。
例句：
Many workers choose to join a trade union to protect their rights. （许多工人选择加入工会以保护他们的权益。）
He decided to become a member of a trade union after learning about its benefits. （在了解到工会的好处后，他决定加入工会。）