Okay…thanks to a friend who sent me this link: https://stuartfarish.com/advanced-custom-fields-sort-by-field-group-sub-field/
All you have to do is add the field name of the group to the beginning of the subfield. The name of my group is showit_event_dates. So, the code should look something like this:
$args = array (
'post_type' => 'showit_events',
'posts_per_page' => $num_posts,
'orderby' => 'meta_value',
'meta_key' => 'showit_event_dates_start_event',
'order' => 'DESC'
);
Hope that helps someone
