function tag_with_name( $user_id, $contact_id, $post_data ) {

    if( ! empty( $post_data['first_name'] ) && ! empty( $post_data['last_name'] ) ) {

        $tag = $post_data['first_name'] . $post_data['last_name'];

        wp_fusion()->user->apply_tags( array( $tag ), $user_id );

    }

}

add_action( 'wpf_user_created', 'tag_with_name', 10, 3 );