In Spanish, the nosotros/nosotras/usted/ustedes forms of the imperative, both in affirmative and negative commands take same form as El Presente de Subjuntivo. Let's look at nosotros/nosotras first, then usted/ustedes.
To say Let's [do something] or Let's not [do something] in Spanish as a form of command to ourselves we use the verb in the present subjunctive form for nosotros/nosotras (first person plural).
Have a look at these examples:
Ella no quiere que nosotros comamos tan rápido. (present subjunctive)
She doesn't want us to eat so fast.
Alternative structures not using the imperative
Bear in mind though, that in a more colloquial way, we would probably use an interrogative sentence introduced by "¿Por qué no...?" followed by El Presente with the same purpose, an affirmative command to "ourselves". For example:
¿Por qué no vamos a la playa?
Why don't we go to the beach?/Let's go to the beach!
And in the negative cases, we would probably prefer to use a different sentence structure introduced by "Mejor no..." followed by El Presente. For example:
Mejor no vamos a la playa.
Let's not go to the beach!
Both "¿Por qué no...?" and "Mejor no..." followed by El Presente are more frequently used in speech than their equivalent forms using the imperative:
Vayamos a la playa.
Let's go to the beach! (affirmative command)
No vayamos a la playa.
Let's not go to the beach! (negative command)
Here are other examples using the imperative:
Usted (you formal singular) and Ustedes (you formal plural)
To express an affirmative or negative command directed to "you" (formal) we also use the usted/ustedes form (3rd person singular/plural) in El Presente de Subjuntivo. A typical situation is in a formal conversation or transaction, for example in a bank.
Here are some examples:
Necesito que firme aquí, Señor. (present subjunctive)
I need you to sign here, Sir.
Señoritas, no es bueno que beban mucho champán. (present subjunctive)
Ladies, it is not good to drink lots of champagne.
Another typical context for this is road signs:
Here are more examples (usted):
Here are more examples (ustedes):