Hi gents, I would be to know if is possible to create a mirror of a group (like an instanced group to maintain the possibility to add new meshes and modify the singles) of meshes because I try with mirror modifier without success. Many thanks! 
Dupligroup http://wiki.blender.org/index.php/Doc:2.6/Manual/Modeling/Objects/Duplication/DupliGroup
Example blend attached
Attachments
dupligroup.blend (224 KB)
Thank you but I know the group instance, what I don’t understand is how I can create an instance directly mirrored in an axis, because I don’t know how move the pivot in a group to make that (like snap that at a left vertex of cube), in object mode I can’t move the pivot of groups snapping to vertex… When I create a normal “mirrored group” with “group instance” Blender create the same group but just translated close to the original, not mirrored. Can you explain the process to make that right? Many thanks!!
(Problem Solved)
You need to change the Pivot Center on 3D Cursor after changed the pivot in edit mode (Mesh > Snap > Cursor To Selected) to snap on a vertex. I forgotten this step. Thank you however!