Not only will the two prior methods work, but you may also save the assembly as an IGES file and then import the IGES file back into ProE. It will ask you if you want to import the IGES as a part or as an assembly, choose part though it is my experience that the IGES still only make as single part, choosing assembly only embeds the part one level deeper. This method can work to hide internal components (possibly company IP) but still providing externally accurate models for publication.