hi
without API u ahve to small changes in ur Table & also on ur User creation form
in ur "user.dbf" u have to add one field "Ustatus" logical
& in ur login form when ur system verify password sucessfully then
u have to update ur "user.dbf" table update Field "Ustatus"=.t.

& ur User creation form u have to add one List box
& write Query check user login status =.t. then u can find currently how many user using ur EXE & system exit time u have to update ur "user.dbf" update field "Ustatus"=.f.

try this. i think this idea will help u

regards
Mazahir