It is a good practice to create new table for your plugin when plugin is activated.

register_activation_hook is used to fire a custom function when a plugin is acttivated. So here is a simple function which will trigger when plugin is get activated.
function create_user_table()
{
global$wpdb;
$charset_collate = $wpdb->get_charset_collate();
$table = $wpdb->prefix.'users';
$sql = "CREATE TABLE IF NOT EXISTS $table
 ( `id` INT NOT NULL AUTO_INCREMENT , 
 `name` TEXT NOT NULL , `password` TEXT NOT NULL ,
 `email` TEXT NOT NULL , PRIMARY KEY (`id`)) 
 ENGINE = InnoDB; $charset_collate;";
require_once(ABSPATH .'wp-admin/includes/upgrade.php');
dbDelta($sql);
}
register_activation_hook( __FILE__, 'create_table' );

Note: register_activation_hook must be call in plugin root file, otherwise it is not going to be work.