Querying multiple categories in the WP loop if one of the category IDs is dynamically obtained

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);