1.take part in的近义词是join in,不是join.
2.join的宾语为“人/团体/组织/机构”,而take part in和join in的宾语都是各种各样的活动如比赛/竞赛/晚会等.
3.如：
1）Can I join you?
I joined the army in 1998.
2）I took part in the competition yesterday.
= I joined in the competition yesterday.