filter.php

<?php
namespace Bitrix\Calendar\Integration\UI\EntitySelector\Attendee;
 
use Bitrix\Calendar\Sharing;
use Bitrix\UI\EntitySelector\BaseFilter;
use Bitrix\UI\EntitySelector\Dialog;
use Bitrix\UI\EntitySelector\Item;
 
class Filter extends BaseFilter
{
    public function __construct(array $options)
    {
        parent::__construct();
        $this->options = $options;
    }
 
    public function isAvailable(): bool
    {
        return $GLOBALS['USER']->isAuthorized();
    }
 
    public function apply(array $items, Dialog $dialog): void
    {
        $eventId = (int)$this->getOption('eventId', 0);
        $isSharing = $this->getOption('isSharingEvent', false);
        if ($eventId > 0 && $isSharing)
        {
            $eventLink = (new Sharing\Link\Factory())->getEventLinkByEventId($eventId);
            if ($eventLink)
            {
                foreach ($items as $item)
                {
                    if (!($item instanceof Item))
                    {
                        continue;
                    }
                    if ($item->getId() === $eventLink->getOwnerId() || $item->getId() === $eventLink->getHostId())
                    {
                        $item->setDeselectable(false);
                    }
                }
            }
        }
    }
}