Possibly something along these lines is what you're after:
<font face=Tahoma><SPAN style="color:#00007F">Private</SPAN> <SPAN style="color:#00007F">Sub</SPAN> Workbook_Open()
<SPAN style="color:#00007F">Dim</SPAN> strUser <SPAN style="color:#00007F">As</SPAN> <SPAN style="color:#00007F">String</SPAN>
<SPAN style="color:#00007F">Dim</SPAN> c
<SPAN style="color:#00007F">Dim</SPAN> FirstAddress
strUser = Environ("USERNAME")
Sheets("Sheet1").Range("A1").Value = strUser
<SPAN style="color:#007F00">' Assuming that you list of users is on a sheet called "Authorized Users"</SPAN>
<SPAN style="color:#007F00">' Test if the Current User is Authorized to use the workbook</SPAN>
<SPAN style="color:#00007F">With</SPAN> Sheets("Authorized Users").Range("A1:A166")
<SPAN style="color:#00007F">Set</SPAN> c = .Find(strUser, 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 strUser & " is authorized for use", vbInformation + vbOKOnly, "Authorized Use"
<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">End</SPAN> <SPAN style="color:#00007F">If</SPAN>
<SPAN style="color:#00007F">If</SPAN> c <SPAN style="color:#00007F">Is</SPAN> <SPAN style="color:#00007F">Nothing</SPAN> <SPAN style="color:#00007F">Then</SPAN>
MsgBox strUser & " is not authorized for use", vbCritical + vbOKOnly, "Unauthorized Use"
<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