Authorization rules for web sites folder

Hi,

I have create a group in Active Directory (ex:  WebUser).  Now i'm going to IIS 7.5, navigate thru a folder in a web sites and click 'Authorization Rules'.

Now here is my problem:

I have create a specific user (ex:  MyName) part of the group WebUser.  If i create an Allow Rules and Specified users with just MyName, it's work, but if i choose Specified roles or user groups with Domain\WebUser, it's not working.

Someone have an answer?