Any criticism should be aimed at ideas, not individuals. Answers alone are not enough. Be prepared and willing to explain your reasoning. Do not be persuaded by group pressure. In mathematics, majority rule is frequently dangerous.
Various roles are useful in effective group-learning.
When you are convinced of your solution, try to explain it carefully. You've really succeeded, if everyone ends up being able to give the explanation. If the logic of an argument is not clear, ask for clarification. If you think there's a mistake or misinterpretation or the team's investigation is headed in the wrong direction, ask appropriate questions to help focus on the error and get the team headed in a more productive direction. Facilitate participation by everyone.