Thanks Truong, yes, I just want to pull posts from a single custom post type "staffmembers" and the code I have I think is more for pulling from any post type.
So change this
function your_prefix_register_meta_boxes( $meta_boxes ) {
to this
function your_prefix_register_meta_boxes( $meta_boxes, $options ) {
Stuck on replacing $post_types = get_post_types( '', 'staffmembers', 'objects' ); even after reading the documentation, though. Goal is a dropdown list of the slugs of the custom post type "staffmembers" as the option value, and if possible the post name as the visible option name. If they need to be the same, that would work for the purposes of the list.