So, I think I’ve got it figured out… …get_cat_ID didn’t seem to be the correct function. I swapped it out with get_category_by_slug and that eventually got it up and running.
//LINK IN BLOG POSTS THAT MATCH CHAPTER SLUG
function chapter_posts() {
//Get Group Slug
$postCategory = bp_get_current_group_slug();
//Get posts
global $post;
$chapterCatID = get_category_by_slug($postCategory)->term_id;
$args = array(
'cat' => ''. $chapterCatID . ',15',
'posts_per_page' => 4
);
$posts = get_posts( $args ); ?>
<div class="group_post_start">
<ul>
<?php
foreach( $posts as $post ): setup_postdata($post);
?>
<li>
<a href="https://wordpress.stackexchange.com/questions/208863/<?php the_permalink() ?>" title="Permanent Link to <?php the_title(); ?>"><h2><?php the_title(); ?></h2></a>
<div class="group_post_display">
<p><?php the_excerpt(); ?></p>
<p><a href="https://wordpress.stackexchange.com/questions/208863/<?php the_permalink() ?>" title="Permanent Link to <?php the_title(); ?>" class="button">Read More</a></p>
</div>
</li>
<?php
endforeach;
wp_reset_postdata(); // reset the query ?>
</ul>
</div><!--.group_post_start-->
<div class="group_post_end">
<a href="<?php echo get_site_url();?>/blog/category/mompreneurs/<?php echo $postCategory; ?>/" class="button">More</a>
</div><!--.group_post_end-->
<?php
}
add_action( 'bp_before_group_home_content', 'chapter_posts', 50);