Add a "member_number" field that only admins can edit using PMPro and Register Helper.

my_pmpro_member_number_field.php

<?php
/**
 * Add a "member_number" field that only admins can edit.
 * Make sure PMPro and the Register Helper Add On are active.
 * Copy this code into a Code Snippet or custom plugin.
 */
function my_pmpro_member_number_field() {
	// Don't break if Register Helper is not loaded.
	if ( ! function_exists( 'pmprorh_add_registration_field' ) ) {
		return false;
	}

	// Define the fields.
	$fields = array();
	$fields[] = new PMProRH_Field(
		'member_number',					// input name, will also be used as meta key
		'text',								// type of field
		array(
			'label'		=> 'Member Number',	// custom field label
			'size'		=> 40,				// input size
			'profile'	=> 'only_admin',	// show in user profile
			'required'	=> false,			// make this field required
           		'memberlistcsv' => true,
            		'addmember' => true
		)
	);

	// Add the fields into a new checkout_boxes are of the checkout page.
	foreach ( $fields as $field ) {
		pmprorh_add_registration_field(
			'checkout_boxes',				// location on checkout page
			$field							// PMProRH_Field object
		);
	}

	// That's it. See the PMPro Register Helper readme for more information and examples.
}
add_action( 'init', 'my_pmpro_member_number_field' );