"Where shall we go next month?"
"Together" might not be in the English sentence but it's definitely implied. The question is whether the same is true in Japanese. Does the omission or inclusion of 一緒に have any affect on how natural the sentence sounds to a native speaker?
In lingodeer 一緒に is used a lot more liberally in these types of sample sentences, which makes me wonder whether Duolingo is simplifying it for this exercise, whether lingodeer is overly complicating it, or if both are as correct/common as each other.
My guess would be that because ましょう (let's/let us/etc. ~) is used, the "we together" is implied.