“How's everyone?”
You're giving a presentation to a group of a few dozen people. You're starting your presentation. You want to start it in a friendly, laid-back way, so you say this.
How's everyone?
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How is everyone?
When you're talking to one or two people, you can ask them "How are you?" to greet them and start a conversation. When you're talking to a larger group, like your audience when you're giving a presentation or speech, you can say this instead:
How is everyone?
or:
How's everyone?
This sounds friendly and casual.
The audience might answer, or might stay quiet when you ask "How is everyone?