Your 2011 files will need to be saved down to an earlier format in order to be opened on the older versions.

But even though the new files can be opened on the older versions, you will most likely experience problems if your 2011 version is Autocad Mechanical and your older versions are just plain Autocad. Vertical products such as Autocad Mechanical make use of special components that Autocad cannot deal with. You can download and install object enablers that will allow Autocad to see the components, but they will not be editable. And since you have to save all the way down to R14, you may end up losing some objects that may not be able to be converted. This is just a guess since I have no way of testing this theory. You could probably explode the components and then be able to edit them in the older versions, but this will make them useless back in 2011 since their functionality will have been removed by the explosion.

My advice would be to upgrade your users and get everyone on Autocad Mechanical. I realize this may be a very expensive proposition, but you may have problems if you don't.