Greeting, Introduction And Offer

Greetings in a casual way, out of office and out of classroom with colleagues and classmates:-

Hi/Hello
e.g. Hi John.

Greeting in the office and classroom:-
Good morning.
Good afternoon.
Good evening.

e.g. Good morning, Marry.

 Self-Introduction:-

Casually:-

e.g. Hello! I am John.

In a office or classroom:-

e.g. Good morning, I am John

Introducing Others:-

Introducing a friend:-

e.g. Hello everybody! Meet my friend, Sam.
Or
This is my friend, Sam.

Introducing a colleague:- 

e.g. Good morning, everybody! I would like to introduce my colleague Mr. Sam. He is with the Finance department.

After being introduced 'Sam' would say:-

Pleased to meet you.
Or
Nice meeting you.
Or
It's a pleasure meeting you.

Others respond to 'Sam' like:-

Pleased to meet you too.
Or
Nice meeting you too.

Offering or Invitation:-

Sam, would you like to join us for a cup of coffee?
Or
Sam, may I take you out for dinner?

If 'Sam' accepts the offer, he would say:-

Yes, thank you.
Or
I would love to, thank you.

If 'Sam' refuses or declines an offer or invitation, he should be polite and could say:-

 That's very nice of you.