In Zukkoke Otokomichi, they translate things like 中ってこうぜ as "Let's ~" Why does こうぜ mean let's? Is it just slang, or does it come from 行こう, just dropping the 行？
こうぜ is just how いこうぜ gets pronounced in lively "tough guy" kind of speech. It is used in group activities such as sports where a team effort is essential.
Te-form of a verb + （い）こうぜ！ = "Let's all (verb), guys!"