If a container is set as the current container, any subsequent nodes you create are automatically added to that container.
You can also   -click
         a container in the Hypergraph Connections editor
         and select Set Container as Current.
 -click
         a container in the Hypergraph Connections editor
         and select Set Container as Current. 
      
You can set the heads-up display to show your current container in the bottom right corner of the scene panel by selecting Display > Heads Up Display > Current Container.