1．take part in
take part in 意为“参加,参与（某事物或某活动,如movement,revolution,meeting,conference,conversation,war等）”.例如：
Switzerland didn't take part in this war.瑞士没有参加这次战争.
How many countries will take part in the World Cup?有多少个国家要参加世界杯?
2．join
join意为“参加（某组织）,加入（某处任职）,参加到某个人群中去,从而成为其中一员”.例如：
She joined a health club.她参加了一个健身俱乐部.
We both joined the Labor Party.我们俩都加入了工党.
3．join in
join in sth./ doing sth.意为“参加”,后面接的宾语一般是表示竞赛、娱乐、游戏等活动的名词或V-ing.例如：
Can I join in the game?我可以参加这个游戏吗?
They all join in singing the song.他们一起唱这首歌.