jperkins is right. Copy Geom is the way to go. If I can make a suggestion, use Copy Geom in conjuction with a Publish Geom. This will help you keep track of external references in your model.
Create a Publish Geom feature in the assembly. The publish geom feature should contain all the references you want to pass to the part. Then create the Copy Geom feature in the part. Specify the Copy Geom feature to reference the Publish Geom feature from the assembly.
You don't have to create the Publish Geom feature. Copy Geom will work without it. But if you or someone else comes back to this model later, you will immediately recognize the external reference between the part and the assembly.