Went do we use "njooni" in this context instead of just using "njoo"?
"Njooni" is plural of njoo - so telling more than one person to come would be njooni but person would be "Njoo"
True, but (at the risk of pointing out the obvious) there is no way of telling from the English whether the singular or plural "you" is intended, so both "njoo" and "njooni" must be accepted as correct answers. BTW, this seems to be a generic problem. The French course has solved this issue. Hopefully the Swahili course will also solve it.