Be honest, helpful, and kind. We all want to succeed, but we make games to enjoy making things for people to enjoy. If we just wanted to make money, we'd ditch all this and go into banking.
After the designer has introduced their prototype, if you are not comfortable participating, let the designer know your concerns. It may not help them much to get feedback from outside their target audience. But your outside perspective could provide useful insight. It's good if you can come to an agreement, but you always have the right to step away.
You don't need to play a full session of a game to playtest it. If you've played for an hour, it's okay to say "I think I'm ready to give you my feedback." But if everyone wants to continue, that's okay too!
Be open with your feedback. You may be a game's first playtester outside the designer's friends and family. They need perspective from helpful and objective people to make a game that more people will enjoy.
Hold your feedback until the game starts. Ask for clarification if necessary, but actually try the game out before offering suggestions. If you can't let the thought go, write it down and come back to it later.
Give your feedback once and be done. Don't get upset if the designer doesn't seem immediately inclined to act on your ideas. Games are complex systems, and you may not understand all the ramifications of your suggestions. Or the designer may have a particular vision that is critical to their interest in the design. Either way, don't repeat yourself: let the next person speak, or the next playtest begin.
If a playtester tells you they see a problem, they're probably right. But it's up to you to decide if the problem is worth what it will take to fix. And it's up to you to decide how to fix it: You don't have to follow every suggestion, although you should understand the reasoning behind it.
It's easy to get caught up in the game design you're testing today. But no painter paints a single painting. No author writes a single story. Maybe focus less on making this one game better, and focus more on learning to become a better game designer. Be willing to set a favorite idea aside for now and try something different.
If you see a designer looking for testers, join their session before pulling out your own prototype. After that session is done, you'll have a group ready for another playtest, and that's a great time to propose yours.
In order for everyone to take a turn, you will need to spend more time playtesting other designers' games than playtesting your own. But you should be able to get at least one solid playtest of your own prototype each day. If that doesn't seem like it's happening, ask a staffer for help.
Whatever happens, you will be spending the day with a lot of fun, helpful, intelligent people. Do what you can to make sure everyone feels heard, and has a chance to do what they came here to do. Grab a meal with someone. Make some new friends. Get home safe, so you can come back and see us again next time.