C# - Get Windows Group Members Along With Their Domain Names?

I have a windows group called "windgrp" it has three members in it:

Administrators
testDomain.Administrator
user1

I have this code to display the members present in a group:

using (DirectoryEntry groupEntry =
new DirectoryEntry("WinNT://./" + userGroupName + ",group"))
{
foreach (object member in (IEnumerable)groupEntry.Invoke("Members"))
{
using (DirectoryEntry memberEntry = new DirectoryEntry(member))
{
listbox.itms.add(memberentry.name);
}
}
}

This gives me the result:

Administrator
Administrator
user

It does not show me to which domain the 2nd entry belongs to.

How can I get the domain?