Translation:What were you discussing just now?
I agree the translation is fine, but I personally would say "what were you just talking about." This Chinese course almost always requires you to put the time word at the end of the sentence, but there should be some flexibility. As the location of the time word can often vary. "what were you just talking about" may not be the best example as it is a little colloquial, but it should still be accepted I think.
I think Duo may be trying to be trying to emphasize the difference between 刚 “just” and 刚才 "just now." Given different contexts, 刚 "just" can indicate a minute ago, an hour ago, or even the day before. However, 刚才 "just now" indicates the immediately preceeding moment.
"You" is used in English to address both singular and plural audiences. Many languages have different words for singular and plural but English does not. Leaving out "guys" is just as correct (in fact, it's arguably better - "you guys" is more colloquial).
"What were you talking about just now?" is 100% correct whether "you" is one person or many.