I figured out a way of doing it by comparing the_date() of each article to the previous one and either creating a new group or not depending on the outcome. No need to edit the Loop at all! 🙂
<?php $prevDate=get_the_date(); ?>
<?php while ($prevDate==get_the_date()): ?>
<li>
<div class="article-thumb"><a href="<?php the_permalink() ?>"> <?php the_post_thumbnail(); ?> </a></div>
<div class="article-title"><a href="<?php the_permalink() ?>"><?php the_title() ?></a></div>
<div class="byline"><?php the_author_posts_link(); ?></div>
<div class="kicker"> <?php the_excerpt(); ?> </div>
</li>
<?php $prevDate=get_the_date(); ?>
<?php the_post(); ?>
<?php endwhile; ?>
