1.join 表示参加;加入;成为...的一员

  He joined the army in 1945 and joined the Communist Party next year.

  他于一九四五年参军，并在第二年入党。

  Will you join us? 跟我们一起来,好吗？

2.join in 表示加入某种活动(游戏,活动,讨论等)

  After that more and more countries joined in the games.

  此后，越来越多的国家参加了奥运会。

  Will you join us in the discussion?

3.join sb. in... 和某人一起加入某种活动

  She'll join us in singing songs.

  她将和我们一起唱歌。

  They joined me in congratulating you.他们与我一道向你表示祝贺。

4.join up 联接起来,联合起来、

  Can you join up the wires? 你能把电线连接起来吗？

  Northern and Southern states joined up again as one country and slaves were set free.