Все, решил проблему. Вот:

myGuid = new Guid(GUIDReestr);
objClassType = Type.GetTypeFromCLSID(myGuid, COMServerReestr);
COMObject = Activator.CreateInstance(objClassType);
COMObject.GetType().InvokeMember("Ping", BindingFlags.InvokeMethod, null, COMObject, null);