I'm trying to get a list of the groups that are in an AD group using .NET.

As an example, I have a group called TestGroup and inside that group I have the group DomainAdministrators.

Using the code below I can get all of the users including those from the DomainAdministrators group but not the group itself.

PrincipalContext ctx = new PrincipalContext(ContextType.Domain, "DomainName");
GroupPrincipal grp = GroupPrincipal.FindByIdentity(ctx, IdentityType.Name, "TestGroup");

ArrayList members = new ArrayList();

if (grp != null)
{
    foreach (Principal p in grp.GetMembers(true))
    {
        members.Add(p.Name)
    }

}   
grp.Dispose();
ctx.Dispose();

Instead of GetMembers I've tried GetGroups but that doesn't return anything. How can I return the groups in the group?