Log User Activity to a separate file

If they enable macros then yes.

You would need to place a call to the logging routine in the Workbook_Open event.

The logging routine can use simple file I/O to add a record to the file

Thisworkbook code

Private Sub Workbook_Open()

    LogOpen
    
End Sub

Module code

Public Function LogOpen()

    Dim intUnit As Integer
    Dim strWindowsLogon As String
    Dim strComputerID As String
    Const LOG_FILE = "C:\temp\log.csv"
    
    intUnit = FreeFile
    Open LOG_FILE For Append As intUnit
    
    Write #intUnit, Now, strWindowsLogon, strComputerID

    Close intUnit
    
End Function

You need to search the forum for details on getting the logon and computer id.