To make the English expressly plural subjects, you would say "You all walk to come here"...without that, there is no indication of singular or plural . Both Temba kuja hapa and Tembeeni kuja hapa are correct.
I have attempted to learn numerous languages over the years and I want to share two linguistic life lessons:
1) no language needs a 2nd person plural. Ever.
2) time spent in the Duolingo comments is time not spent learning a language.
I completely disagree on both accounts even though I'm also a linguist. Second person plural is an amazing feature, English is very poor for not having it. Comments on Duolingo are often more valuable than the exercices themselves.
Njoo is the same verb as kuja, but to use njoo would be like saying, "walk come here" because njoo is a command form. Instead, the command in this sentence is tembee/tembeeni, and the verb "come" is not a command but an infinitive: kuja =to come