join / join in / take part in / attend
1) join表示“加入党派，组织、社团、俱乐部”等。如：join the Party/Youth League/army/club/organization入党/入团/参军/加入俱乐部/加入组织。
Would you join us(in) singing?
和我们一起唱歌吧！
His brother joined the army a year ago.
他哥哥一年前参军了。
connect和join的区别join还可表示“来（去）和某人呆在一起，把……连在一起”
I will join you in a few minutes.
我一会就过来。
Please join the two ends of the rope together.
把绳子两头接起来。
2) join in表示“参加正在进行的活动”。如：join in    a game /discussion/conversation/walk/talk参加游戏/讨论/谈话/一起散步/一起讨论。亦可说：join sb. in (doing)sth.表示“加入某人一起做某事”。
3) take part in表示“参加会议、活动”，侧重说明主语参加并发挥一定作用，part 前若有修饰语，要用不定冠词。如:take (an active)part in a party/school activities/physical labour(积极)参加聚会/学校活动/体力劳动。
4) attend表示“出席、参加会议、仪式、婚礼、葬礼、典礼、上课、上学、听报告”。如：attend a meeting/a sports meeting/a concer/a show/school/a lecture参加会议/运动会/出席音乐会/出席展览会/上学/听演讲。
例句：
They danced and danced until a lot of us joined in.
他们不停地跳着舞, 直到我们中间许多人都参加了进去。
Do you know how many of them joined in the research project?
你知道他们中间有多少人参加了这项研究项目吗?
Would you join us in a walk?
你不同我们一起散步吗?
John takes part in many school activities.
约翰参加很多学校里的活动。
Joe took an active part in the struggle.
乔积极参加斗争。
The whole sales force attended the conference.
整个销售界的权贵都出席了这次会议。
All children are required to attend school.
所有的孩子都必须上学。
His son joined the army last year.
他儿子去年入伍。