Translation:I met you when you were walking
If we are are going to translate this "when" then I think it should be a -po-. Niliwakuta mlipotembea. (Just my thoughts... I am still learning, will always be learning.... )
it should be them not you(niliwakuta) even though you can figure out they are probably talking about you because of the 2nd part(mkitembea) I thinkyou plural plural would be niliwakuteni even though in Tanzania they sometimes leave off the eni if it is obvious that it is you plural. ex. Mungu awabariki(God bless them, but it is often said for God bless you when talking to a crowd.)
Why is the -wa- infix (they) used instead of the -ku- infix (you), or even the -m- infix (you all)?