The users doesn’t have permalinks nor titles. They have author links and display names. Changed the code:
<?php while ( have_posts() ) : the_post(); ?>
<h2>Resturants</h2>
<?php
$resturants = get_users(array( // Changed from get_posts
'fields' => 'all_with_meta', // All with meta to get also display_name
'meta_query' => array(
array(
'key' => 'resturant', // name of custom field
'value' => '"' . get_the_ID() . '"',
'compare' => 'LIKE'
)
)
));
?>
<?php if( $resturants ): ?>
<ul>
<?php foreach( $resturants as $resturant ): ?>
<li>
<a href="https://wordpress.stackexchange.com/questions/188014/<?php echo get_author_posts_url( $resturant->ID ); ?>"><?php echo $resturant->display_name; ?></a>
</li>
<?php endforeach; ?>
</ul>
<?php endif; ?>
<?php endwhile; ?>