Public speaking is very difficult. And in spite of that knowledge, most audiences are very cruel. Most of the time they don't mean to be cruel, but they don't know their own strength. So, if you are going to be in an audience for someone you know who is speaking in public - this is what you do: listen. Don't shout out, don't distract, don't do all of the things that many people think are helping. Just be quiet and listen intently. Smile, make eye contact with the speaker, nod a little bit. No, don't wave your hands or give a big "thumbs up". The reason for this is something that many people never realize - the speaker can see you.
As a public speaker I came to realize that people in the audience think that the public speaker, although right in front of them, can't see them. But if you've ever been in front of a group, you know that you can. In fact, you can see everything, from people turning to talk to each other, to shuffling nervously, to just plain walking away. And that's just human nature. You can't change that. Since the vast majority of people in audiences have never, and will never, speak in public, you can't expect them to know any of that. So this is what you do -
Find one person in the audience to speak to. You don't always have to be staring at this person, but this is the one you are communicating to. Be sure it's someone who likes you, who is interested in what you are saying. It's a trick, but it's a valid trick. In fact, most people in the audience, in spite of the fact that they really don't know how to behave, are really interested in you, and what you have to say. Sure, they may be looking at their watches or picking their nose, but if they knew that you could see them, they wouldn't do it. If you were talking to each and every person there one-on-one, their behavior would be different. But in a crowd, most people imagine that they are invisible.
Smile. Be confident. Get to the point. Whatever you do, don't try to teach manners to your audience. Imagine that most people in the audience like you, even if they don't know how to show it.