Dear Customer,

From you description, I understand that you want to get a DirectoryEntry's
domain name and its group name.
If I misunderstood, please feel free to let me know.

Based on my research, we need to parse the Path property to get the domain
name and use the Name Property to get the group name.

Here is the code snippet for your reference.
       private void button1_Click(object sender, EventArgs e)
       {
    //You need to change the LDAP path according to your scenario.
           DirectoryEntry de = new
DirectoryEntry("LDAP://CN=Group1,DC=Test,DC=com");
           MessageBox.Show(de.Name);
           Regex rg = new Regex(@"DC=(\w+)");
           string dn="";
           foreach( Match m in rg.Matches(de.Path))
               dn += m.Groups[1].Value+".";
           MessageBox.Show(dn);
       }

You may have a try and let me know the result.
If you still have any concern, please feel free to let me know.
I look forward to hearing from you.

Best regards,

Peter Huang
Microsoft Online Partner Support