Filter webdav access by group

Hello,

We use IIS on Windows 2008 Server R2 x64 for each user can access to their home directory with webdav.

All works fine except if we want to filter access by authorization rules with groups.
If we use rules with users login the filter works great, but for all users in our organization it's simply unmanageable.

We tried filter access with group by "webdav authorization rules" and "classic" authorization rules but in both case it's the same result...

Any idea on why filter by group not work ?

Regards,