when using add_user_to_blog getting error “Call to undefined function get_userdata”

get_userdata() is a pluggable function, you can find its declaration in wp-includes/pluggable.php. That means, plugins can declare it earlier, and it is not declared, when a plugin is loaded.

When you look at wp-settings.php, you can see the load order:

// Load active plugins.
foreach ( wp_get_active_and_valid_plugins() as $plugin )
    include_once( $plugin );
unset( $plugin );

// Load pluggable functions.
require( ABSPATH . WPINC . '/pluggable.php' );
require( ABSPATH . WPINC . '/pluggable-deprecated.php' );

// Set internal encoding.
wp_set_internal_encoding();

// Run wp_cache_postload() if object cache is enabled and the function exists.
if ( WP_CACHE && function_exists( 'wp_cache_postload' ) )
    wp_cache_postload();

/**
 * Fires once activated plugins have loaded.
 *
 * Pluggable functions are also available at this point in the loading order.
 *
 * @since 1.5.0
 */
do_action( 'plugins_loaded' );

Wait at least for plugins_loaded, before you do more than calling add_action() or add_filter().