Proposes doing something together: 갑시다 "let's go", 같이 해 봅시다 "let's give it a try together", 무승부로 합시다 "let's call it a draw". It sounds formal and slightly commanding, which suits announcements, hosts addressing an audience, and speaking to a group — but it is not respectful enough to use toward an elder or a boss, where 시죠 or a question form is safer. It attaches to vowel stems; consonant stems take 읍시다. The casual equivalent is 자 (가자 "let's go").
Register: formal-sounding; not for elders or superiors