This is huge. It’s so important.
If you don’t speak up when you agree, then you’re opening the door for a single dissenter to appear like the majority.
Whether it’s in a formal meeting, or just a “friendly” discussion over lunch with a group of people; affirm and agree when you like what’s being said. Let the others know that you agree. Explain why you personally agree.
When you do this, it helps keep the tone of the conversation/meeting positive. It keeps the cards on the table so to speak. When you don’t speak-up, it creates a tone of secrecy and keeping cards close to your chest. It allows people to (unhelpfully) think the worst, and assume that you do not agree.
So if you do agree, agree out loud.