Most grammar books only introduce the formation of imperatives, but not its use. But when thinking about it in some detail you can notice that the imperative is not only for orders, but has many other functions as well. Still, don't worry the German imperative works in its use the same way like the English one.
Have a look at these phrases
a) Do it! Now!
b) Do it if you want to.
The phrase a) is an order. One person gives an order to another one that is supposedly a subordinate. Phrase b) uses the same form but in a different sense. There is no order, but only a statement.
In German as well as in English an imperative can only be used for some persons, as it is quite clear that one can hardly give an order to oneself. As for the first person plural there are even two forms.
Machen wir es!
Let us do it!
Lasst es uns machen!
Let us go!
Lasst uns gehen!
The word lassen can be used as a auxiliary to form an imperative, but also as a full verb to express that somebody should not do something. Lasst ihn jetzt in Ruhe, und zwar sofort! Let him in peace!... In English there is the same construction, so that you don't have to worry too much neither about forms nor about use.