Hi. The only difference between, "Hi everybody" and "Hi everyone" is , "everyone" is more formal. But both are correct greetings. You can also say, "Hi all". To be "officially proper" English, you would insert a comma thusly: "Hi, everybody." "Hi, everyone." "Hi, all." But mostly in America, especially in an informal context such as conversation with friends, we wouldn't bother with the "officially proper" comma! :) We usually just say, "Hi!" I hope this helps. Good luck!