"Let's meet tomorrow!"
that's what i thought, thank you. I asked because 만납시다 was marked incorrect (i will flag next time)
-ㅂ시다 is formal polite, but there are two formal polite levels and this is the lower one, 하오체. You don't want to say it to your elders. There is a propositive ending in 합쇼체, but it is very rarely used and doesn't even sound very polite, so we decided not to include it in the course. You normally just ask a question instead to your elders, like "Would you do ddd?"
"내일 만날 거예요" is in the future tense, and means "will meet tomorrow." 내일 만나요 is propositive, so it implies the proposition "let's!"