1

If a meet you (singular), I say "你好".

If I meet you (plural), shall I say "你们好", instead?

3

You may also use: 诸位好!or 各位好!

There may be more, Chinese has a lot of ways to express things!

2

大家好 is more common, methinks. 你好 is more for personal interaction, and not very common in group settings.

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