A few other tips related to assemblies:
You can create a child "component" by right-clicking on the root (or other) component and selecting "New Component". This gives you a target to drag and paste.
Once you have more than the root component, you can drag bodies between them.
You can have multiple instances of the same component -- use copy&paste. (Only within a single document.)
--Jon Balgley (Autodesk)
Jon Balgley