Sub-class of League\OAuth2\Server\Grant\AbstractGrant that overrides League\OAuth2\Server\Grant\ClientCredentialsGrant so that clients with an assigned user_id using this grant can have that claim inserted into the JWT.

ClientCredentialsGrant.php

<?php namespace My\Space;

use League\OAuth2\Server\Entities\ClientEntityInterface;
use League\OAuth2\Server\Entities\UserEntityInterface;
use Laravel\Passport\ClientRepository;
use Laravel\Passport\Bridge\User;
use League\OAuth2\Server\RequestEvent;
use League\OAuth2\Server\Exception\OAuthServerException;
use League\OAuth2\Server\Grant\AbstractGrant;

use League\OAuth2\Server\ResponseTypes\ResponseTypeInterface;
use Psr\Http\Message\ServerRequestInterface;

/**
 * Client credentials grant class.
 */
class ClientCredentialsGrant extends AbstractGrant
{
    /**
     * {@inheritdoc}
     */
    public function respondToAccessTokenRequest(
        ServerRequestInterface $request,
        ResponseTypeInterface $responseType,
        \DateInterval $accessTokenTTL
    ) {
        // Validate request
        $client = $this->validateClient($request);
        $scopes = $this->validateScopes($this->getRequestParameter('scope', $request));
        $user = $this->validateUser($request, $client);

        // Finalize the requested scopes
        $scopes = $this->scopeRepository->finalizeScopes($scopes, $this->getIdentifier(), $client);

        // Issue and persist access token
        $accessToken = $this->issueAccessToken(
            $accessTokenTTL,
            $client,
            ($user ? $user->getIdentifier() : null),
            $scopes
        );

        // Inject access token into response type
        $responseType->setAccessToken($accessToken);

        return $responseType;
    }

    /**
     * @see League\OAuth2\Server\Grant\PasswordGrant::validateUser
     */
    protected function validateUser(ServerRequestInterface $request, ClientEntityInterface $client)
    {
        $clientRecord = app()->make(ClientRepository::class)->findActive($client->getIdentifier());
        if(!$clientRecord->user_id) {
            return null;
        }

        $user = new User($clientRecord->user_id);
        if ($user instanceof UserEntityInterface === false) {
            $this->getEmitter()->emit(new RequestEvent(RequestEvent::CLIENT_AUTHENTICATION_FAILED, $request));

            throw OAuthServerException::invalidCredentials();
        }

        return $user;
    }

    /**
     * {@inheritdoc}
     */
    public function getIdentifier()
    {
        return 'client_credentials';
    }
}