Add product at user registration

/**
 * Basic action to set a default product at registration
 * 
 * Set product slug accordingly. Example is "slug_of_the_product".
 */
add_action( 'wpmem_post_register_data', function( $fields ) {
    global $wpmem;
    $wpmem->user->set_user_product( 'slug_of_the_product' );
});

/**
 * Set a product based on field selection.
 *
 * In this example, the product is set based on the value of
 * a select field (dropdown) where the select saved values 
 * are equal to the product slugs.
 * 
 * Also, adjust field meta key according to the actual field
 * used - example is "product_selection".
 */
add_action( 'wpmem_post_register_data', function( $fields ) {
    global $wpmem;
    $wpmem->user->set_user_product( $fields['product_selection'] );
});