<?php
add_filter( 'rwmb_meta_boxes', 'nested_groups_demo' );

function nested_groups_demo( $meta_boxes )
{
    // Meta Box
    $meta_boxes[] = array(
        'title' => __( 'Books', 'rwmb' ),

        'fields' => array(
            array(
                'id'     => 'authors',
                'name'   => __( 'Authors', 'rwmb' ),
                'type'   => 'group', // Group type
                'clone'  => true,

                // List of child fields
                'fields' => array(
                    array(
                        'name' => __( 'Full Name', 'rwmb' ),
                        'id'   => 'name',
                        'type' => 'group',

                        'fields' => array(
                            array(
                                'id'   => 'first_name',
                                'name' => __( 'First Name', 'rwmb' ),
                                'type' => 'text',
                            ),
                            array(
                                'id'   => 'last_name',
                                'name' => __( 'Last Name', 'rwmb' ),
                                'type' => 'text',
                            ),
                        ),
                    ),
                    array(
                        'name' => __( 'Phone', 'rwmb' ),
                        'id'   => 'phone',
                        'type' => 'text',
                        'size' => 10,
                    ),
                    array(
                        'name' => __( 'Email', 'rwmb' ),
                        'id'   => 'email',
                        'type' => 'email',
                        'size' => 15,
                    ),
                ),
            ),
        ),
    );

    return $meta_boxes;
}