The standard meaning of pitch is to toss something lightly, like a ball. In this meaning, when you pitch an idea, it is like tossing it. Usually, when you have an idea you have to pitch it to someone and hope that they like the idea. Example; "You have great ideas, you should pitch them to someone who can help you out."

It means to offer ideas for consideration. It comes from baseball; you pitch (throw) the ball and the batter misses or hits it. In the same way, you offer an idea and people like it or not.

I'd add only, that it does not have to be a group. You can pitch ideas at one person. The concept is the same.

Ideas may be discussed in a group or a meeting at work.