Simple Staff List Add Custom Field Example - does not add to [simple-staff-list] output, just creates a metabox and saves the data.

sslp-post-nominal-field.php

<?php
/**
 * Plugin Name: Simple Staff List - Post Nominal Field
 * Description: Adds a "Post Nominal" field for Simple Staff List plugin
 * Version: 1.0
 * Author: Brett Shumaker
 * Author URI: https://brettshumaker.com
 * License: GPL2
 **/


/**
 * Notes:
 * This will only add a custom metabox for the staff-member post type
 * and save that data. It _WILL NOT_ add a way to get your new field
 * into the [simple-staff-list] shortcode output.
 */

class SSLP_Post_Nominal_Field {

	public function __construct() {

		add_action( 'do_meta_boxes', array( $this, 'add_meta_boxes' ) );

		add_action( 'save_post', array( $this, 'save_post_nominal_field' ), 10, 2 );

	}

	public function add_meta_boxes() {

		add_meta_box(
			'sslp-post-nominal-field',
			'Post Nominal',
			array( $this, 'meta_box_callback' ),
			'staff-member',
			'side'
		);

	}

	public function meta_box_callback( $post ) {

		wp_nonce_field( 'sslp_post_nominal_field', 'sslp_post_nominal_field' );
		$value = get_post_meta( $post->ID, '_staff_member_postnominal', true );

		echo '<label for="_staff_member_postnominal">Post Nominal</label><br />';
		echo '<input name="_staff_member_postnominal" id="_staff_member_postnominal" value="' . esc_attr( $value ) . '" />';

	}

	public function save_post_nominal_field( $post_id ) {

		// Verify the nonce field.
		if ( ! isset( $_POST['sslp_post_nominal_field'] ) || ! wp_verify_nonce( $_POST['sslp_post_nominal_field'], 'sslp_post_nominal_field' ) ) {
			return;
		}

		// Don't autosave this metadata
		if ( defined( 'DOING_AUTOSAVE' ) && DOING_AUTOSAVE ) {
			return;
		}

		// Make sure the user can edit the post
		if ( ! current_user_can( 'edit_post', $post_id ) ) {
			return;
		}

		if ( isset( $_REQUEST['_staff_member_postnominal'] ) ) {

			update_post_meta( $post_id, '_staff_member_postnominal', sanitize_text_field( $_POST['_staff_member_postnominal'] ) );

		}

	}

}

new SSLP_Post_Nominal_Field();