1.sign up for

sign up for意思报名参加,签约参加工作, 签约雇用

2.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?

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。