It is my understanding that in 2012 Revit, I cannot save a file down as a2011 Revit rvt file. What if the other companies I am sharing files withare not using 2012 but using 2011?

You need to use the same version as your cooperating teams. Almost all parametric 3D CAD modelers cannot save back to older file formats.
 
Alternatively, you might try the IFC exhange format (export/import).