I've read about getting it with the Environment class, but can't find it.
Thanks guys.
A way based on the Jono response, but shorter:
public static string GetWorkGroup()
{
ManagementObject computer_system = new ManagementObject(
string.Format(
"Win32_ComputerSystem.Name='{0}'",
Environment.MachineName));
object result = computer_system["Workgroup"];
return result.ToString();
}