For implode() to work, the second argument has to be an array, but I do not see the variable $user_info set anywhere.
Now, since there could be anything behind that variable, please dump it and post what it spits out, provided that it does not reveal any sensitive information:
<h3>Asistentes</h3>
<div>
<?php $users = get_field('asistentes'); ?>
<ul>
<?php
foreach ($users as $user):
var_dump( $user , $user_info );
?>
<li>
<p><?php echo $user['display_name'];
echo 'User roles: ' . implode(', ', $user_info['role']) . "\n";?></p>
</li>
<?php endforeach; ?>
</div>
