But we can upgrade your code =))
<?php
global $current_user;
get_currentuserinfo();
if (in_array(explode(' ',get_field('mustjoin')),$current_user->user_login)) { ?>
<?php while ($day['events']->have_posts()) : $day['events']->the_post() ?>
<?php tribe_get_template_part('month/single', 'event') ?>
<?php endwhile; ?>
<?php } ?>
