Try putting this code in the workbook's _Open() event handler. To do so,
open the workbook, then right-click on the Excel icon right next to the word
File in the standard menu and choose [View Code] from the list. copy the
code below and paste it into the module shown to you and close the VB Editor.
Close the workbook and see how it works.
Essentially it's looking for the name of a sheet in the workbook, not
necessarily true initials, but you don't have to tell the users that. It is
not case sensitive, so JLL is same as jll, is same as Jll, etc. If you
mistype it, it gives you infinite chances to get it right, but if you leave
it empty, it will close the workbook.
Private Sub Workbook_Open()
Dim userInitials As String
Do While userInitials = ""
userInitials = InputBox("Enter your user Initials:", "To Continue...", "")
If userInitials = "" Then
ThisWorkbook.Close ' bye bye!
End If
On Error Resume Next
Worksheets(userInitials).Activate
If Err <> 0 Then
Err.Clear
MsgBox "Those are not valid initials"
userInitials = ""
On Error GoTo 0
End If
On Error GoTo 0
Loop
End Sub