what's the difference between take part in and join in?

take part in的主语是活动的主要参加者（主体）：Three teams took park ( in the game).不谈具体参加内容时，不用in短语。

join 和join in的意思是，在别人进行活动时的加入者（客体）：We are playing basketball .Come and join us.

join的宾语一般是人、团体、组织等；join in的宾语一般是活动。