Instancing works rather simply. Let’s see how in this example scene: 

Select the node(s), right click and choose Group Selection. 

Once you have the group, select Save Compound And Link.

Now, you can have the same Group multiple times in the scene if you drag them in. For example:

They are now instances.If you modify one of the groups, all of the other groups will change accordingly.

To modify you can:

1. Open the group as a compound, do the necessary modifications, then save it. 

OR 

2. Open any of the already existing groups.

Edit it.

And when done, get back to the root, right-click the modified group and select Save In-Place Modifications.

If done correctly, all the groups should change after saving.

Make sure the other instances are not opened (the nodes have to have the chain icon on their top left corner) otherwise they will not change.

Please note that this instancing technique is not exactly the same as the instancing in modeling softwares.
You can achieve similar things, but on one hand this instancing is more general, on the other hand model instancing is usually used for performance reasons, but node instancing only makes your life easier.