A node group is a collection of one or more nodes. You 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.
Node groups are primarily used to ensure that the nodes in a cluster are in the same group. For example, if node "a" is in NodeGroupA and node "b" is in NodeGroupB, these nodes cannot be members of the same cluster.