Solved. This is what I did. In order to not make heavy loads i enebleded Entity Construction Kit (eck) module, then by rules i added an entity (instead of a node) for every user entity reference value, within two columns, one for user name, and one for date in attendance. Now I can have a view (of entities) with two exposed filter on dates (beginning and ending dates) Then I override my views table tpl view with the following code. It displays a 2 column table, first: username, second: number of days that user has attendance. the following code assumes that username is second field on your view ($fieldloop == 2 ). code:
<?php $names = array(); ?>
<?php foreach ($rows as $row_count => $row): ?>
<?php $fieldloop = 1; ?>
<?php foreach ($row as $field => $content): ?>
<?php if ($fieldloop == 2 ) {$names[] = strip_tags($content);} ?>
<?php $fieldloop = $fieldloop + 1; ?>
<?php endforeach; ?>
<?php endforeach; ?>
<?php $attendances = (array_count_values($names)); ?>
<body>
<table>
<tr><th>Alumno</th><th># dias</th></tr>
<?php foreach ($attendances as $attendance => $value) : {
?><tr><td> <?php print $attendance ; ?></td>
<td> <?php print $value; ?></td></tr>
<?php } endforeach; ?>
</table>
</body>