一般接某项活动。例如：I hope everyone will join in the fun.我希望每个人都能参与这个娱乐活动。join in作“参加，加入”讲时，一般指参加某项活动，join作“参加，加入”讲时，一般指参加某一组织、团体或某群人。

相关介绍：

1、He started to sing and I joined in. 他开始唱起歌来，然后我也跟着唱起来。

2、You don't have to join in; there are more than enough people on the job already. 人够多了，您就不用插手了。

3、Now and again he'd join in when we were playing video games. 我们玩电子游戏时，他偶尔会和我们一起玩。

4、I didn't want to join in, but Kenneth insisted. 我不想加入，但是肯尼思非要坚持。

5、I did my best to join in, but the conversation was largely unintelligible. 我努力想插上话，可是大部分谈话内容我都听不懂。