A node group is a collection of one or more nodes. You can view, add, update, or delete node groups at System Administration > Node Groups. By default, when a node is federated into the dmgr, the node will be placed in DefaultNodeGroup.
Selecting a node group and then Node group members will display the nodes in the node group. In this example, the node group has 3 members, the dmgr and two application server nodes that were federated into the dmgr.
Probably the most common use of a Node Group is to control the application servers that will be members of a Dynamic Cluster. When creating a new dynamic cluster, you can setup a rule that will query the application servers that are in a Node Group. In this example, the application servers that are in DefaultNodeGroup will be members of the dynamic cluster.