1. Let's do sth. ---

Let's All Go Together 唱片名

Let's all sit down.  我们都坐下来。

2. We are going to do sth. ---

We are all taking medicine against the cold. 

我们都服药以防感冒。

We are all connected in the great circle of life. 

在这个巨大的生命圈里面，我们都是互相有关联的。

all of us —— of us 作定语。