add the following to the addanother.php page at line 1 to force the page to see the session.
<?php @session_start(); ?>
actually, you should not be relying on the session variable on the add another page.
What happens if the user closes the browser? the session will get cleared at that point, and adding another wil fail.
instead, on the members page, you should show a listing of members and have a link for each memebr to add another that passes the member ID as a URL variable to the addanother page.
store the URL variable to a hidden form element, and use the hidden form element in the insert.