If you feel strongly about this, submit a bug report with the "Report a problem" button.
Personally, I want them to start using a "you all", "you-all" or "y'all" version, to allow us to differentiate in English between singular and plural "you" ... but none of these options is really standard/official English, so I don't know if it should actually be built into the course. Even your "you guys" version is kind of colloquial ... but then again, none of us in this course are trying to learn better English.
Does anyone have any thoughts on this issue of clarifying between singular and plural "you"? Especially any of the course developers?
As a student, having a you(pl) that I could answer with would help me keep the plural vs. singular distinction straight in my head. Because of where I live, I would default to y'all personally, but I would be fine with a different option as well.
As Arcaeca points out, it would be useful to have the formalities of the different options clarified as well.
Even though I don't have y'all in my dialect and it kind of grates on my ears (especially when used for two people), I'm behind it. It would be very useful here and it's not ambiguous like any other option in English.
No idea how to indicate formality without adding extra words like "you sir/madam" ...