Add an organization field to the 'checkout_boxes' area of the checkout page.

pmpro-organization-register-helper.php

<?php
function add_organization_pmpro_RH_fields() {
	if ( ! defined( 'PMPRO_VERSION' ) || ! defined( 'PMPRORH_VERSION' ) ) {
		return;
	}
	$fields = array();

	$fields[] = new PMProRH_Field(
		'organization_name',  // Input name becomes usermeta key, so no hyphens or spaces 
		'text',               // The type of field to use
		array(
			'label'           => 'Organization name', // Display Label
			'size'            => 40,                  // Size of the input field
			'class'           => 'rh-organization',   // Custom CSS class (your choice of name)
			'profile'         => true,                // Include user's profile (true | false | 'only' | 'only_admin')
			'memberslistcsv'  => true,                // Included in "Member List" CSV export (true | false)
			'addmember'       => true,                // Used if the "Add Member Admin" add-on is present (true | false)
			'required'        => false,               // Make this field required (true | false)
		)
	);

	/**
	 * This part adds the fields into 'checkout_boxes' area of the checkout page. There
	 * are other areas you can add to.
	 */
	foreach ( $fields as $field ) {
		// Add the field(s) to the checkout page.
		pmprorh_add_registration_field(
			'checkout_boxes', // location on checkout page
			$field            // PMProRH_Field object
		);
	}
}
add_action( 'init', 'add_organization_pmpro_RH_fields' );