opensupports/server/libs/validations/validOwnersId.php

<?php

namespace CustomValidations;

use Respect\Validation\Rules\AbstractRule;

class ValidOwnersId extends AbstractRule {

    public function validate($owners) {
        if(is_array(json_decode($owners))){
            foreach (json_decode($owners) as $owner) {  
                $author = \Staff::getDataStore($owner);
                if($author->isNull()) return false; 
            }
            return true;
        }
        return false;
    }
}