We all had different experiences when ideating with groups. Some more pleasant than others. There is one common lousy practice we probably all suffered: that moment when your colleagues kill your ideas right at the start.
There is a simple way of reasoning why this won't benefit you or the outcome of your creative session.
Let's imagine you are a football coach. You obviously want to win the game. What would be a better tactic, to try to shoot to goal as many times as possible, or get just one chance and score it? The answer is obvious: the more you shoot, the more probabilities you'll score. Simple math.
The situation is the same when ideating. In the beginning, you need a lot of ideas. As many as possible. The more you get, the more possibilities of having a great one.
It's not merely about getting the right idea, but it's also about getting away the pressure from your colleagues. If everyone expects you only to contribute when the concept is magnificent, you'll feel under pressure and most likely talk much less.
Ideas should flow freely at the beginning. Use them as an inspiration to build on top of them. Treat them as they are: seeds for something bigger.