guys

In English, there's not a really good word for addressing a group of people. If you're talking to one person you say "you":

How are you?

But there's some disagreement about what to say to a group of people. English speakers use different expressions depending on the region and dialect, but one common expression is "guys". For example, at a restaurant with a large group of friends you can say:

Hey guys, are you ready to order?

"Guys" technically means a group of men, but it's usually OK to use it for mixed groups of men and women, or even for groups of all women.

In very formal situations, avoid calling a group of people "guys". Instead, use a phrase like "everyone":

Everyone, thank you for coming.

let's (do something)

Make a suggestion using "let's ___" when you're pretty sure that the people you're speaking to will accept your suggestion. If you're not as sure, you can say "why don't we ___":

Guys, why don't we call it a day.

call it a day

This phrase, "Call it a day", means to stop what you're doing for that day.

You use this when you're not 100% finished with everything you need to do, but you're tired or ready to go home. Use "Call it a day" when you're working on projects with people, practicing for something together, or just to let people know when you're leaving work:

Jean, I'm going to call it a day. I'll see you tomorrow.