[statement] + 呗
[verb] + 就 + [verb] + 呗
呗 goes at the end of a spoken sentence and presents it as the obvious answer – there's nothing more to it. Depending on the voice it sounds relaxed, resigned or a bit dismissive.
Verb + 就 + verb + 呗 accepts something you didn't choose: "fine, then I'll do it".
On a request between friends, 呗 makes it light and easy to say yes to.
呗 never marks a guess; that is 吧. And because it can sound offhand, it doesn't go in a reply to a teacher or someone you don't know well.