join in多指参加某组织，成为其中的一个成员，指参加到某些人当中去做某事，这项活动往往正在进行着；take part in指参加会议或者群众性的活动等，重在说明句子的主语参加该项活动并在活动中发挥作用。

take part in例句

The officer refused to let his men take part in the operation.

这军官拒绝让他的士兵参加这项军事行动。

Six theatre companies have been selected to take part in this year's festival.

You don't need strength to take part in this sport

参加这项运动不需要太多体力。

They returned to take part in the season's opening game.

他们回来参加本赛季的开幕赛。

You can take part in activities from canoeing to bird watching.

你可以参与从划独木舟到观鸟等各种活动。

join in例句

I hope that everyone will be able to join in the fun

我希望每个人都能够加入进来，玩得开心。

You still have time to join in the fun.

现在加入其中享受快乐还来得及。

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。

No matter how often they were urged, they could not bring themselves to join in

就算别人说破了嘴皮，他们就是不加入。

The main rebel groups have agreed to join in a meeting to set up a transitional government.

主要叛乱集团已同意参加会议讨论建立一个过渡政府。