I think they do not show it on charts because it is very simple and there is nothing special in it: 1. Sprich! 2. Sprecht! 3. Sprechen Sie, bitte! 4. Sprechen wir! / Wollen wir sprechen!

My German teacher told use you can use "lassen" as a Hilfsverb, OR that you could also say something like "gehen wir!".

I'm curious - how WOULD you form the imperative for "wir"? Or for "ich" or the 3rd person singular, for that matter? It's impossible, isn't it? And isn't it for exactly the same reason that it's impossible to form the imperative in English for any other person than "you"? You could only form the imperative "Write!" for either the 2nd person singular or the 2nd person plural, same as in German. German just adds the in-this-case-not-so-polite form "Schreiben Sie!", which corresponds to the 3rd person plural, but that's it, isn't it?

For a German, which never had the opportunity to look at these charts, which form do you mean? The kind which begins with "Lasst uns [Verb] machen!"? If that's the form you mean, then maybe it is not shown on the charts, because of the use of the verb "lassen", which is grammatically used with an imperative for second person plural, as the speaker is, again purely grammatically, telling everyone else to allow doing it together.