Change button text . Registration Form

you could try the string ovverides module or try to do some coding like this
http://mydrupal.com/customize_user_login_box_block

In a custom module add this code

function yourmodule_form_alter(&$form, &$form_state, $form_id) {
  if ($form_id == 'user_register') {
    $form['submit']['#value'] = 'Complete the Join Process';
  }
}