Maybe something like this:
<font face=Tahoma><SPAN style="color:#00007F">Private</SPAN> <SPAN style="color:#00007F">Sub</SPAN> Workbook_Open()
<SPAN style="color:#00007F">Dim</SPAN> CurrentUser <SPAN style="color:#00007F">As</SPAN> <SPAN style="color:#00007F">String</SPAN>, FirstAddress <SPAN style="color:#00007F">As</SPAN> <SPAN style="color:#00007F">String</SPAN>
<SPAN style="color:#00007F">Dim</SPAN> c <SPAN style="color:#00007F">As</SPAN> Range
CurrentUser = LCase(Environ("UserName"))
<SPAN style="color:#00007F">With</SPAN> Sheets(1).Range(Cells(1, 1), Cells(Rows.Count, "A").End(xlUp))
<SPAN style="color:#00007F">Set</SPAN> c = .Find(CurrentUser, LookIn:=xlValues)
<SPAN style="color:#00007F">If</SPAN> <SPAN style="color:#00007F">Not</SPAN> c <SPAN style="color:#00007F">Is</SPAN> <SPAN style="color:#00007F">Nothing</SPAN> <SPAN style="color:#00007F">Then</SPAN>
FirstAddress = c.Address
<SPAN style="color:#00007F">Do</SPAN>
MsgBox "Authorized to Proceed"
<SPAN style="color:#00007F">Set</SPAN> c = .FindNext(c)
<SPAN style="color:#00007F">Loop</SPAN> <SPAN style="color:#00007F">While</SPAN> <SPAN style="color:#00007F">Not</SPAN> c <SPAN style="color:#00007F">Is</SPAN> <SPAN style="color:#00007F">Nothing</SPAN> And c.Address <> FirstAddress
<SPAN style="color:#00007F">Else</SPAN>
MsgBox "Unauthorized to Proceed"
<SPAN style="color:#007F00">' ActiveWorkbook.Close False</SPAN>
<SPAN style="color:#00007F">End</SPAN> <SPAN style="color:#00007F">If</SPAN>
<SPAN style="color:#00007F">End</SPAN> <SPAN style="color:#00007F">With</SPAN>
<SPAN style="color:#00007F">End</SPAN> <SPAN style="color:#00007F">Sub</SPAN></FONT>
Smitty