Hi Neil!
You can to use a cursor and use it in ListBox. Eg.:

SET EXCLUSIVE OFF
SET DELETED ON

Create Cursor curUser (userName C(30), userNo I)

Select users.userno, users.username ;
  from "U:\Questpdm\Data\Users" ;
  ORDER BY users.username ;
  into array arrUsers

if ( _Tally > 0 )
for ix = 1 to alen(arrUsers,1)
  If File("U:\QuestPDM\Profile\User"+ltrim(str(arrUsers[ix,1]))+"\insystem.dbf")
    lnFID = Fopen("U:\QuestPDM\Profile\User"+ltrim(str(arrUsers[ix,1]))+"\insystem.dbf", 12)
    If lnFID = -1	&& Get users are in system
       Insert Into curUser (userName, userNo) Values(arrUsers(ix, 2), arrUsers(ix, 1))
    ENDIF
  ENDIF
 ENDFOR
ENDIF

I hope this help you.