There was a file format change in the 2013 release, just as there was with the 2010, 2007 and 2004 releases. If you open a file created in one file format in a release that uses a later file format, the AEC objects are "promoted" to the new file format. If you then save that file in the later release, there is no way for earlier-format releases to make use of AEC objects, even if you "save back" to the earlier format. In fact, if your version of ACA detects future versions of AEC objects, all of the AEC modules will be disabled for that file, and you will not be able to even create new ones.
With proxy objects turned on by the 2013 user, you could open and view the file in an earlier release, but you would not be able to edit AEC objects or run AEC commands. The 2013 user could EXPORT the file to the file format you use (2010, for ACA 2011), but that would explode the AEC objects to AutoCAD primitives.
Bottom line, if you need to share files with a team, all team members need to be using releases of the same file format.