USERNAME

USERNAME()
Returns either the application user name or the network domain name.

REMARKS
The application username can be seen from the (Tools > Options)(General tab).
The network username can be seen from ..
UserName("Application") = "Matthew Smith"
UserName("Domain") = "smithm"

Public Declare Function GetUserName Lib "advapi32.dll" Alias "GetUserNameA" (ByVal lpBuffer As String, nSize As Long) As Long 

Public Function USERNAME( _ 
         ByVal sType As String) _ 
         As String 

Dim sBuffer As String * 255 
Dim lLength As Long 
Dim sUserName As String 

   If (UCase(sType) = "APPLICATION") Then 
      UserName = Application.UserName 
   End If 
   
   If (UCase(sType) = "DOMAIN") Then 
      sUserName = "" 
      lLength = GetUserName(sBuffer, 255) 
      lLength = InStr(1, sBuffer, Chr(0)) 
      If (lLength > 0) Then 
          sUserName = Left(sBuffer, lLength - 1) 
      Else 
          sUserName = sBuffer 
      End If 

      USERNAME = UCase(Trim(sUserName)) 
    End If 
End Function 

For instructions on how to add this function to a workbook refer to the page under Inserting Functions