Wordpress - Adding fields to the "Add New User" screen in the dashboard

user_new_form is the hook that can do the magic here.

function custom_user_profile_fields($user){
  ?>
    <h3>Extra profile information</h3>
    <table class="form-table">
        <tr>
            <th><label for="company">Company Name</label></th>
            <td>
                <input type="text" class="regular-text" name="company" value="<?php echo esc_attr( get_the_author_meta( 'company', $user->ID ) ); ?>" id="company" /><br />
                <span class="description">Where are you?</span>
            </td>
        </tr>
    </table>
  <?php
}
add_action( 'show_user_profile', 'custom_user_profile_fields' );
add_action( 'edit_user_profile', 'custom_user_profile_fields' );
add_action( "user_new_form", "custom_user_profile_fields" );

function save_custom_user_profile_fields($user_id){
    # again do this only if you can
    if(!current_user_can('manage_options'))
        return false;

    # save my custom field
    update_usermeta($user_id, 'company', $_POST['company']);
}
add_action('user_register', 'save_custom_user_profile_fields');
add_action('profile_update', 'save_custom_user_profile_fields');

For more details visit my blog post: http://scriptbaker.com/adding-custom-fields-to-wordpress-user-profile-and-add-new-user-page/

I had the same need and created the following hack:

<?php
function hack_add_custom_user_profile_fields(){
    global $pagenow;

    # do this only in page user-new.php
    if($pagenow !== 'user-new.php')
        return;

    # do this only if you can
    if(!current_user_can('manage_options'))
        return false;

?>
<table id="table_my_custom_field" style="display:none;">
<!-- My Custom Code { -->
    <tr>
        <th><label for="my_custom_field">My Custom Field</label></th>
        <td><input type="text" name="my_custom_field" id="my_custom_field" /></td>
    </tr>
<!-- } -->
</table>
<script>
jQuery(function($){
    //Move my HTML code below user's role
    $('#table_my_custom_field tr').insertAfter($('#role').parentsUntil('tr').parent());
});
</script>
<?php
}
add_action('admin_footer_text', 'hack_add_custom_user_profile_fields');


function save_custom_user_profile_fields($user_id){
    # again do this only if you can
    if(!current_user_can('manage_options'))
        return false;

    # save my custom field
    update_usermeta($user_id, 'my_custom_field', $_POST['my_custom_field']);
}
add_action('user_register', 'save_custom_user_profile_fields');

You need to do 2 things.

1. Register fields
2. Save fields

Note: Below example works only for administrator user role.

---

1. Register fields

For Add New User use action user_new_form

For User Profile use actions show_user_profile, edit_user_profile

Register fields Snippet:

/**
 * Add fields to user profile screen, add new user screen
 */
if( !function_exists('m_register_profile_fields') ) {

    //  This action for 'Add New User' screen
    add_action( 'user_new_form', 'm_register_profile_fields' );

    //  This actions for 'User Profile' screen
    add_action( 'show_user_profile', 'm_register_profile_fields' );
    add_action( 'edit_user_profile', 'm_register_profile_fields' );

    function m_register_profile_fields( $user ) {

        if ( !current_user_can( 'administrator', $user_id ) )
            return false;

        ?>

        <h3>Client Portal</h3>
        <table class="form-table">
            <tr>
                <th><label for="dropdown">Portal Category</label></th>
                <td>
                    <input type="text" class="regular-text" name="portal_cat" value="<?php echo esc_attr( get_the_author_meta( 'portal_cat', $user->ID ) ); ?>" id="portal_cat" /><br />
                </td>
            </tr>
        </table>
    <?php }
}

---

2. Save fields

For Add New User use action user_register

For User Profile use actions personal_options_update, edit_user_profile_update

Save fields Snippet:

/**
 *  Save portal category field to user profile page, add new profile page etc
 */
if( !function_exists('m_register_profile_fields') ) {

    //  This action for 'Add New User' screen
    add_action( 'user_register', 'cp_save_profile_fields' );

    //  This actions for 'User Profile' screen
    add_action( 'personal_options_update', 'cp_save_profile_fields' );
    add_action( 'edit_user_profile_update', 'cp_save_profile_fields' );

    function cp_save_profile_fields( $user_id ) {

        if ( !current_user_can( 'administrator', $user_id ) )
            return false;

        update_usermeta( $user_id, 'portal_cat', $_POST['portal_cat'] );
    }
}

Complete Code Snippet:

/**
 * Add fields to user profile screen, add new user screen
 */
if( !function_exists('m_register_profile_fields') ) {

    //  This action for 'Add New User' screen
    add_action( 'user_new_form', 'm_register_profile_fields' );

    //  This actions for 'User Profile' screen
    add_action( 'show_user_profile', 'm_register_profile_fields' );
    add_action( 'edit_user_profile', 'm_register_profile_fields' );

    function m_register_profile_fields( $user ) {

        if ( !current_user_can( 'administrator', $user_id ) )
            return false;

        ?>

        <h3>Client Portal</h3>
        <table class="form-table">
            <tr>
                <th><label for="dropdown">Portal Category</label></th>
                <td>
                    <input type="text" class="regular-text" name="portal_cat" value="<?php echo esc_attr( get_the_author_meta( 'portal_cat', $user->ID ) ); ?>" id="portal_cat" /><br />
                </td>
            </tr>
        </table>
    <?php }
}


/**
 *  Save portal category field to user profile page, add new profile page etc
 */
if( !function_exists('m_register_profile_fields') ) {

    //  This action for 'Add New User' screen
    add_action( 'user_register', 'cp_save_profile_fields' );

    //  This actions for 'User Profile' screen
    add_action( 'personal_options_update', 'cp_save_profile_fields' );
    add_action( 'edit_user_profile_update', 'cp_save_profile_fields' );

    function cp_save_profile_fields( $user_id ) {

        if ( !current_user_can( 'administrator', $user_id ) )
            return false;

        update_usermeta( $user_id, 'portal_cat', $_POST['portal_cat'] );
    }
}