This will record a user's ID each time Excel file is opened:
Private Sub Auto_Open()

   LogInformation ThisWorkbook.Name & " opened by " & _
   Application.UserName & " " & Format(Date, "yyyy-mm-dd") & " " &
Format(Time, "hh:mm")
   'LogInformation Variable1 & "," & Variable2 & "," & Now
   
End Sub

Sub LogInformation(LogMessage As String)
Const LogFileName As String = "C:\MyLog.txt"
Dim FileNum As Integer
   FileNum = FreeFile ' next file number
   Open LogFileName For Append As #FileNum ' creates the file if it doesn't
exist
   Print #FileNum, LogMessage ' write information at the end of the text file
   Close #FileNum ' close the file
End Sub

Notice, you must have a Text file named 'MyLog.txt' at this location:
"C:\MyLog.txt"

Regards,
Ryan--