Doh. Never mind, I got it working. Seems like inserting another add_post_meta line but removing the true argument did the trick.
//Set Default Meta Value
function set_default_meta_new_post($post_ID){
if ($current_field_value == '' && !wp_is_post_revision($post_ID)){
add_post_meta($post_ID,'groups-read','2');
add_post_meta($post_ID,'groups-read','4');
}
return $post_ID;
}
add_action('wp_insert_post','set_default_meta_new_post');
Cheers