Active Directory Membership and Role Provider - Get user name

[wild_bill]

I'm using the built in membership (System.Web.Security.Membership) and role (System.Web.Security.Roles) providers with active directory. I would like to get the user's name that is stored in active directory. When I use MembershipUser.UserName, I get their login id. Right now I run their email (MembershipUser.Email) through an alorithm to get their name, but this seems hokey. My second problem is that I need to determine all the users in a specific role. My first thought was to iterate through all the users, then determine if they are in the role I need:

For Each u As MembershipUser In Membership.GetAllUsers
    If Roles.IsUserInRole(u.UserName, "InformationManagementGroup") Then
        userList.Add(New UserInformation(u.UserName, u.Email))
    End If
Next

When I try this approach, I get an error: Method is only supported if the user name parameter matches the user name in the current Windows Identity.

All help is greatly appreciated.