"Let's meet tomorrow!"
-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?"