Group nodes are used to create sub-trees within the overall conversation tree.  This is useful from an organizational standpoint but also becomes very valuable in defining conditions as you will see in defining condition priorities. Groups can be thought of as simplified dialogue nodes and contain some of the same fields.  Group nodes can also be added, arranged, and linked just like dialogue nodes.  As of Chat Mapper version 1.7, scripts defined on group nodes will be executed just like script assigned to dialogue nodes.

Group Nodes In A Conversation