Prayer Circles

What are “prayer circles”? I have heard of entire congregations getting handed a book on it.  It seems to be growing in groups that I thought were otherwise basically conservative.  I heard it was even used and promoted at a True Woman Conference about a year ago.[1]  What is its appeal?  Should Reformed Christians be concerned about this idea and why? The idea of Prayer Circles was popularized by a recent book called The Circle Maker[2] by Mark Batterson of National Community Church,...