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.

Hey (name)

You say "hey" to people who you know when you first see or speak to them:

Hey Boris.

"Hey" is the same as "Hello" but more casual.

How's it going?

There are basically two meanings to the phrase "How's it going":

1. Hello.
2. How are you?

If someone who you know asks "How's it going?" while walking past you or working on something busily, it's similar to saying "Hi". You can respond with:

Hey, how's it going?

Good morning.

Fine, thanks!

If you're sitting down with someone and they ask "How's it going?", then they're trying to start a conversation. You should answer the question and add some other information or questions:

A: How's it going?

B: Not bad. I've been so busy this week, though. It's crazy. How are you doing?

"How's it going?" is an abbreviation for "How is it going?".

This phrase is a little casual, but it's OK in a lot of different situations. If you want to sound more formal, you can say "Hello" to greet a person, or "How are you?" to start a conversation.