Since today is a Sunday, group prayer seems like a good topic.
With the prayer closet, you only have yourself, God, and the spirit world as your listeners. With group prayer you have other humans listening in, and adding their own silent prayers to yours.
If you are the listener, you should do your best to follow along and add your own silent thoughts and affirmations to the prayer being spoken. An occasional "Amen!" or "Yes, Lord," can be added out loud, but excessive amounts of that can be distracting and detract from the prayer being said. You might ask yourself whether or not (only God and you know) you are adding such phrases to glorify God and edify fellow worshippers or show off how spiritual you are.
I will address the topic of saying a group prayer out loud in the next post.