Log/Record every user of a workbook

No problem.

Where do you want the log of who's done what and is there any need for user security to prevent people from manually editing the log?

To identify who has opened the workbook:

Public Function requestor(q As Integer) As String
    Set objAD = CreateObject("ADSystemInfo")
    Set objUser = GetObject("LDAP://" & objAD.Username)
    requestor = objUser.DisplayName
End Function

This will pull the network user name (full name, not UID). You can reference this by putting "=requestor(1)" into any cell on any worksheet.

Then, in the thisworkbook module put code to log this somewhere using the before_close event.