add_existing_user_to_blog( array $details = false )

function add_existing_user_to_blog( $details = false ) {
	if ( is_array( $details ) ) {
		$blog_id = get_current_blog_id();
		$result = add_user_to_blog( $blog_id, $details[ 'user_id' ], $details[ 'role' ] );

		/**
		 * Fires immediately after an existing user is added to a site.
		 *
		 * @since WP-MU (3.0.0)
		 *
		 * @param int   $user_id User ID.
		 * @param mixed $result  True on success or a WP_Error object if the user doesn't exist
		 *                       or could not be added.
		 */
		do_action( 'added_existing_user', $details['user_id'], $result );

		return $result;
	}
}