Are they really all men? If they are, "gentlemen" is fine. If they aren't, "Ladies and gentlemen" will do.
There's nothing wrong with some formality, especially in business or any other professional setting. More people should observe it. It represents good manners, something else more people should display.
You want people to take a meeting or presentation seriously. Formality helps.
If you really want to get casual, use "Folks". I don't like it, but it would do.