The following code will accomplish what you are seeking:

Public Sub CJPHX()

Dim arrInitials(3) As String
Dim i As Integer
Dim blnfound As Boolean

blnfound = False
arrInitials(0) = "CJH"
arrInitials(1) = "PHT"
arrInitials(2) = "FXH"
arrInitials(3) = "TOM"

Sender = InputBox("Please Enter You Initials:")
For i = 0 To UBound(arrInitials())
    If arrInitials(i) = Sender Then blnfound = True
Next i

If blnfound = False Then
    MsgBox "Invalid Initials"
    Exit Sub
End If

End Sub
You could also set up a custom form with a listbox with all your expected user's initials, and let them select them from the list.