Duolingo tends to disregard punctuation, and tends to favor the standard over informal. In everyday conversation with tone of voice to provide context, "You guys want dinner?" is perfectly fine. But Duolingo is a computer program and can't really tell the difference between that as a question and that as a statement.
Are you asking me out on a date, Duo? If so, squeeeeee Uh, I mean to say - I accept, casually and totally not desperately. I've actually been dreaming of this mo- oh, well only if you've got nothing else to do. Oh you have? Well, another time mayb- Or not, whatever works best for you my lo- Oh, no I understand. Silly really, as if you'd ever want want to date m- You're right, I am a loser, I'll be single forev- Yes, Duo, I do need to grow some backbone for the next time I ask a bird out. Waits till he's flown off WHYY DUOOOO? WHY WON'T YOU LOVE ME? I'D DO ANYTHING!