Both worked great!
Just to share, this is what I came up with on my own:
Private Sub Workbook_Open()
Dim rng As Range
lastrow = Sheet1.Range("A" & Rows.Count).End(xlUp).Row 'gets number of last used cell
Set rng = Range("M2:M" & lastrow)
With rng.Validation
.Delete
.Add Type:=xlValidateCustom, AlertStyle:=xlValidAlertStop, Operator:= _
xlBetween, Formula1:= _
"=IF(ISERROR(VLOOKUP(M2,IDs!$A$2:$A$65536,1,FALSE)),FALSE,TRUE)"
.ErrorMessage = "INVALID DATA!"
End With
End Sub
May not be the best out there but works just the same
I used workbook_open() because my friend's file is in xls