For discussion and questions about the Reusable Node Groups documentation page, found at http://www.embrio.io/documentation/reusable-node-groups

There is only one copy of each agent in a program and all agents are always running. When you drag an agent onto another node screen you are making a link to the agent.

A node group is a template of a group of nodes. When you drag a node group onto a node screen a whole copy of the groups nodes are made and encapsulated by a single node. This is just a handy way of not duplicating work.

So when you create a new node group file, no nodes are added to the actually run time program until you drag that node group from your project tree into an agent's node screen. When you create a new agent, any nodes you add are already part of your program and will execute at run time.