This is quite interesting to know that who is accessing your shared folder in network. We can easily get the information of that user and even you can track what is the activity he is doing in your system. We can achieve this using WMI (Windows Management Instrumentation) and MQL ( Management Query Language ) .

For using WMI you need to add one DLL called System.Management.dll and add the namespace as

using System.Management

Now Read the current Session of your system.

private  void Read_Current_Session()
        {
           try
            {
                ManagementObjectSearcher searcher =
                   new ManagementObjectSearcher("root\\CIMV2",
                   "SELECT * FROM Win32_ServerConnection");
                foreach (ManagementObject queryObj in searcher.Get())
                {
                   String RemoteIPAddress = queryObj["ComputerName"].ToString();
                   String RemoteUserName = queryObj["UserName"].ToString();
                   String  RemoteActiveTime = queryObj["ActiveTime"].ToString();
                }
            }
            catch (ManagementException e)
            {
               MessageBox.Show("WMI Error: " + e.Message);
            }
        }

Few Years back I have developed one Open Source Tool (NetSpY) based on WMI and MQL. NetSpy (NetWorkSpy) is an Small Windows based application which will continiouly monitor your shared folder in the network and will generate the complete log of the accessing user activities. This can show you the immidiate popup message that who is accessing right now.

If you are interested please have a look over here,
Net Spy: Your Network Spy That Monitor Your System In Network And Generate Log For Any Changes