Attendee fields

functions.php

<?php
$fields_api->register_field(
        new Charitable_Campaign_Field(
            'attendees',
            [
                'label'          => 'Attendees',
                'data_type'      => 'meta',
                'admin_form'     => [
                    'required'  => false,
                    'section'   => 'hike-details',
                    'type'      => 'content',
                    'base_path' => __DIR__,
                    'view'      => 'attendees-admin-field.php'
                ],
                'campaign_form'  => [
                    'required'  => false,
                    'section'   => 'campaign-details',
                    'type'      => 'content',
                    'priority'  => 20,
                    'fullwidth' => true,
                    'content'   => 'content goes here'
                ],
                'show_in_export' => true,
            ]
        )
    );

Note: In the admin_form section I set it up so you can create a view file for the field in the admin. To do that, add the above code to the Charitable Hikes plugin you have, and add a new file called attendees-admin-field.php.

In that field, you can get the array of attendees like this:

global $post;

$campaign = new Charitable_Campaign( $post );
$attendees = $campaign->get( 'attendees' );