どちら様ですか？って英語でなんて言うの？

Both of these questions are a proper way to ask in a business setting.

Hello- will get their attention. "May I ask who you are?" Asks them to tell you their name.

Hello, I am (your name) - will get their attention and introduces yourself. "And you are?" Will prompt them to introduce themselves.

you are?, means who are you and what is your name.

1. Here you introduce yourself and explain that you believe this person to be a new employee. Unless this person is socially inept, they will introduce themselves.
   Socially inept = such a person either has little skill in socialising, or is clumsy in the way that they do.
2. In these days of security and terrorism, everyone expects to be challenged if it is not obvious that they are authorised to be in a place such as a business office or manufacturing plant. Usual forms of politeness do not apply here.
3. A direct challenge to the person concerned to state his reason for being in that location.

A: Hello! I'm Denton. Who are you?
B: I'm Kelly. I am new here.

A: Hi there! What is your name?
B: I'm Susan. Who are you?

It is quite in order to ask some one to formally "introduce themselves..."
In a friendly manner ...we would start by giving our own name...and then ask the stranger to identify themselves, (by name and position...)
"Hi there stranger! My name is Scobie, please introduce yourself."