tricky!
What you can try is to use a markerfile (with just one number, ie the number
of current users).
Look under help under "Write" and "Get" how to manage input files.
If you include some VB code in Workbook_Open and Workbook_BeforeClose then
you can use the markerfile as a counter.
Something like

Type Record    ' Define user-defined type.
   ID As Integer
   Name As Integer
End Type

Dim MyRecord As Record, Position    ' Declare variables.

' Open sample file for random access.
Open "TESTFILE" For Random As #1 Len = Len(MyRecord)
' Read the sample file using the Get statement.
Position = 1    ' Define record number.
Get #1, Position, MyRecord    ' Read third record.

'which will give the variable MyRecord the number in the file
'Now check
if MyRecord > 4 then
    msgbox("to many users, file will autoclose")
    application.close
end if

Write #1, MyRecord + 1

Close #1    ' Close file

in the Workbook_BeforeClose include a similar statement....