Everything about disagreeing changes once an audience is watching, not just what you say, but how the room reads what happens next, and how the person you disagreed with feels about being challenged in front of others.
The core content of the disagreement doesn't need to change in a group setting. What needs to change is the framing, because a group disagreement can easily read as a challenge to someone's standing in the room, not just a difference of opinion about the topic. The goal is to make it clear you're disagreeing with an idea, not attacking the person who raised it, especially if that person outranks you.
"I don't think that's going to work. We tried something like that before and it didn't go well."
"I want to flag a concern based on something similar we tried last year, it ran into an issue with X. Curious if this approach accounts for that differently, or if I'm missing context."
The second version raises the exact same substantive concern, but it does it in a way that leaves room for the other person to respond without losing face, either they have an answer you didn't know about, or they genuinely hadn't considered it, and either way the conversation stays collaborative instead of adversarial.
In a group, how you disagree becomes part of the story people remember. Make sure it's the part where you were useful, not the part where you made someone look bad.
Timing in a group setting matters more than in a one-on-one. Raising a strong disagreement the moment an idea is floated can shut down a room before other people get a chance to react, sometimes it's more useful to let a few other reactions surface first, then add your concern as part of a genuine discussion rather than the first and loudest objection.
If the disagreement is significant enough that you're worried about the public dynamic, disagreeing with your own manager in front of their peers, for instance, it's often worth a quick, quiet flag in the room ("can we take a closer look at this before we finalize it") followed by the real substance of the disagreement in a private follow-up. That protects the relationship while still making sure the concern gets heard by the people who need to hear it.