Okay, never mind. I figured it out. Here is my working code in case anyone has a similar problem.
<?php
if( have_rows('episode_staff') ):
while ( have_rows('episode_staff') ) : the_row();
$user_id = get_sub_field('episode_staff_member')[ID];
?>
<div class="masthead-single">
<h4 class="masthead-name"><?php echo get_sub_field('episode_staff_member')['user_firstname'] ?> <?php echo get_sub_field('episode_staff_member')['user_lastname'] ?></h4>
<ul>
<li><?php echo get_sub_field('episode_staff_member')['user_email'] ?></li>
<li><?php echo get_field('twitter_username', 'user_'. $user_id) ?></li>
</ul>
</div>
<?php endwhile;
endif; ?>
