1. join 的宾语一般是sb或组织团体等,表示成为其中的一员。 如:Please join us in the game! I joined the club last year.
2. join in约等于take part in,他们的宾语都是如比赛/竞赛/考试等各种各样的活动,其中take part in进一步强调在活动中充当一定的角色并发挥一定的作用。 如:I joined in/took part in the English competition yesterday.
两个常用短语: join sb in sth; take an active part in sth=take part in sth actively