namespace Drupal\brqc_custom\Plugin\search_api\processor;

use Drupal\search_api\Datasource\DatasourceInterface;
use Drupal\search_api\Item\ItemInterface;
use Drupal\search_api\Processor\ProcessorPluginBase;
use Drupal\search_api\Processor\ProcessorProperty;

/**
 * Adds a custom field to the indexed data.
 *
 * @SearchApiProcessor(
 *   id = "group__members",
 *   label = @Translation("Group Members"),
 *   description = @Translation("Add a custom field of group members to search index."),
 *   stages = {
 *     "add_properties" = 0,
 *   },
 *   locked = true,
 *   hidden = false,
 * )
 */
class GroupMembers extends ProcessorPluginBase {

  /**
   * machine name of the processor.
   * @var string
   */
  protected $processor_id = 'group__members';

  /**
   * {@inheritdoc}
   */
  public function getPropertyDefinitions(DatasourceInterface $datasource = NULL) {
    $properties = array();

    if (!$datasource) {
      $definition = array(
        'label' => $this->t('Group Members'),
        'description' => $this->t('Custom field for storing all members that belong to this group.'),
        'type' => 'string',
        'processor_id' => $this->getPluginId(),
      );
      $properties[$this->processor_id] = new ProcessorProperty($definition);
    }

    return $properties;
  }

  /**
   * {@inheritdoc}
   */
  public function addFieldValues(ItemInterface $item) {
    $entity = $item->getOriginalObject()->getValue();
    $entity_type = $entity->getEntityTypeId();

    // Use $entity to get custom field.
    // Determine the correct content type.
    if ($entity_type == 'group') {
      // Get members from the group. Temporarily using superhero secret identities.
      $members = [
        'John Doe',
        'Peter Parker',
        'Bruce Wayne',
        'Clark Kent',
        'Diana Prince',
        'Arthur Curry',
        'Jack Napier',
        'James Howlett',
      ];
      $temp_members = [];
      $random = rand(0, count($members));
      $member_keys = array_rand($members, $random);
      if (!is_array($member_keys)) {
        $temp_members[] = $members[$member_keys];
      }
      else {
        foreach ($member_keys as $member_key) {
          $temp_members[] = $members[$member_key];
        }
      }
      $members = $temp_members;

      // Add the group members in individually.
      if (count($members) > 0) {
        $fields = $this->getFieldsHelper()
          ->filterForPropertyPath($item->getFields(), NULL, $this->processor_id);
        foreach ($fields as $field) {
          foreach ($members as $member) {
            $field->addValue($member);
          }
        }
      }
    }
  }

}

{
  "response": {
    "numFound": 1,
    "start": 0,
    "docs": [
      {
        … stuff deleted
        "sm_group__members": [
          "John Doe",
          "Peter Parker",
          "Bruce Wayne",
          "Clark Kent",
          "Diana Prince",
          "Arthur Curry",
          "Jack Napier",
          "James Howlett"
        ],
       … stuff deleted
      }
    ]
  }
}

{
  "data": {
    "searchAPISearch": {
      "count": 1,
      "facets": [
        {
          "name": "group__type",
          "values": [
            {
              "count": 1,
              "filter": "!"
            }
          ]
        },
        {
          "name": "group__members",
          "values": [
            {
              "count": 1,
              "filter": "Arthur Curry"
            },
            {
              "count": 1,
              "filter": "Bruce Wayne"
            },
            {
              "count": 1,
              "filter": "Clark Kent"
            },
            {
              "count": 1,
              "filter": "Diana Prince"
            },
            {
              "count": 1,
              "filter": "Jack Napier"
            },
            {
              "count": 1,
              "filter": "James Howlett"
            },
            {
              "count": 1,
              "filter": "John Doe"
            },
            {
              "count": 1,
              "filter": "Peter Parker"
            }
          ]
        }
      ],
      "documents": [
        {
          … stuff deleted
          "group__members": "John Doe",
          … stuff deleted
        }
      ]
    }
  }
}