In English, greetings like Hello and Hi and Good Morning don't change if you are addressing one, a few, or thousands of people. It's quite common to add something to note that you are greeting many people, such as "Hello everyone" or "Good morning everyone" and certainly there is a range of formality in what you choose to put there.
To my ear, most-to-least formal would be:
- Good morning/afternoon/evening
- Welcome (this also carries an implication that you are the host and "in charge" of the get-together
- Hi there
- Howdy / Hey
And for the group-indicator, again most-to-least formal:
- [List of titles such as Madam Speaker, Sir Thompson, Mr President], [categories with adjectives such as honoured judges, dear guests], ... , ladies and gentlemen (this particular choice for the group-indicator allows you to omit hello/hi entirely and just launch into the list, but it requires you to end with ladies and gentlemen so that people know you have finished listing)
- Ladies and gentlemen
- friends / folks / team
Avoid mixing and matching: both hey there, ladies and gentlemen, let's rock! and S'up, honoured guests? are kind of weird. Not impossible to use, but definitely an expert technique. It's fine to say Good morning folks though.