Best to use when
It is best to do a Group Critique after a Silent Critique has already been done. It allows for in-depth discussion around the idea. It gives time for people to say what they thought was good about the idea and for the person that had the idea to add any more detail to the thought.
Everyone gathers around a sketch or Story Board
Ask people what they liked about it.
Ask the person that had the idea to add any more explanation that they think hasn't been covered or if something was misunderstood.