Kuja is the infinitive literally meaning "to come." While Njoo is the imperative saying "come." So if I'm narrating a situation, and say that you are coming, I will choose to say "Akram anakuja." If I am instead telling you, with authority, to come to me I will say "Akram njoo." Typically you will hear it as a phrase "njoo kwanza" in a real life situation.