It is VBA code and goes in the VBE in the ThisWorkbook module (Alt+F11), paste this in and add more cases as needed:

Private Sub Workbook_BeforeClose(Cancel As Boolean)
Dim Sh As Worksheet
For Each Sh In Sheets
Sh.Visible = True
Next Sh
End Sub
Private Sub Workbook_Open()
Dim Sh As Worksheet
Select Case Environ("username")
Case Is = "Fred"
Sheets("Sheet1").Visible = True
Sheets("Sheet1").Select
Case Is = "John"
Sheets("Sheet2").Visible = True
Sheets("Sheet2").Select
Case Is = "Simon"
Sheets("Sheet3").Visible = True
Sheets("Sheet3").Select
End Select
For Each Sh In Sheets
If Sh.Name <> ActiveSheet.Name Then
Sh.Visible = xlSheetVeryHidden
End If
Next Sh
End Sub