Most verbs change into commands like this
Kupika = pika
But for kuja it is
Kuja = njoo
It's just an irregular form
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.
Kuja- infinitive- "to come" - also building block for present and future tense - ninakuja, hatutakuja, etc
Njoo - command, imperative - "come!"
So surprised right now... been in kenya for two months and literally everyone is using "kuja" as a command also ^-^
Kenyan Swahili seems to be quite diffferent. Technically this is a Tanzanian Swahili course as that is considered "standard" Swahili, but it's very useful to know what to expect in other places. These changes may end up in standard Swahili too.