Question

they let us stand by the door 改成被动语态

Answer 1

在这句话里，let 是不及物动词，是不能将原句用let 改成被动语态的，需要换一个动词，保证原句意思上不变就可以了。
被动：we are asked to stand by the doors (by them). by them. 可写也可不写，没有影响。
只有及物动词才有被动语态，所以在改句子时，如遇到不及物动词，就应换个与之意思相近的及物动词（可能有好几个，选一个，原句意思不变即可）才可以改为被动语态。

Answer 2

We were left standing by the door by them.

Answer 3

we are asked to stand by the door,