I’m not sure exactly what you’re looking to do, but if you want your expiration settings to be set automatically for Buddypress registrations, you can put this code in your functions.php file:
function expire_users_bp_register_form() {
if( class_exists( 'Expire_Users' ) ) {
echo '<input type="hidden" name="expire_users" value="auto" />';
}
}
add_action( 'bp_before_registration_submit_buttons', 'expire_users_bp_register_form' );
It will apply your expiration settings once the user has activated their Buddypress membership.