public function aroundGetCustomerGroupId(
        \Magento\Quote\Model\Quote $subject,
        \Closure $proceed
    ) {
        /* call parent to get group ID and to process other around-plugins */
        $result = $proceed();
        if ($result == \Magento\Customer\Model\GroupManagement::NOT_LOGGED_IN_ID) {
            /* check referral code in registry for guest visitors */
            $code = $this->hlpReferral->getReferralCode();
            if ($code) {
                /* return referral group id if code exists */
                $result = $this->hlpConfig->getReferralsGroupReferrals();
            }
        }
        return $result;
    }