In a sentence "What's up, my friends?" and "Hey, everyone!" should I put a comma between "What's up" and "friends" and "Hey" and "everyone" respectively? Is there a particular set of rules?
When a person/persons are addressed directly in a sentence, the nouns so used, proper or common, are called nouns of direct address. The second person pronoun, "you," can also be used to address someone directly, as can the indefinite pronouns "everyone" or "everybody." Nouns/pronouns of direct address are always set off by commas, regardless of where they fall in the sentence.
And that, my friends, is what is wrong with Congress today.
It's time to eat, Grandma! (Try this sentence without the comma!)
John, your taxi is waiting.
Look, you, don't try that again unless you want the toaster to blow up in your face.
Hey, everyone, Maryanne's brought a chocolate cake.