Translation:Zij zien jullie.
I was told that Ze tot ziens works? Can anyone explain this to me?
Tot ziens is used as a fare well, most similar to the use of goodbye in English. Hence, ze tot ziens does not make any sense to me.
Why did it mark me wrong when I put in "Hun zien jullie?"
'Hun' is not a personal pronoun, it's either 'zij' or 'ze'.
Why did they mark me wrong when I wrote Ze zien jij? because jij is you and it says they see you not they see you guys....?
No, it's because jij can only be the subject of a sentence, have a look at this: https://www.duolingo.com/comment/3852773