Scusate la domanda parecchio ignorante, ma che cambia se faccio:

CREATE TABLE `#__users` (
`user_id` mediumint(8) unsigned NOT NULL AUTO_INCREMENT,
`user_name` varchar(20) NOT NULL DEFAULT '',
`user_password` char(128) NOT NULL DEFAULT '',
`user_salt` char(10) NOT NULL DEFAULT '',
`user_email` varchar(80) NOT NULL DEFAULT '',
`user_avatar` tinytext NOT NULL,
`user_avatar_type` enum('url', 'upload', 'gallery') NOT NULL DEFAULT 'url',
`user_avatar_width` varchar(3) NOT NULL DEFAULT '',
`user_avatar_height` varchar(3) NOT NULL DEFAULT '',
`user_group` tinyint(3) unsigned NOT NULL DEFAULT '0',
`user_hidden` enum('0', '1') NOT NULL DEFAULT '0',
`user_reg_date` int(10) unsigned NOT NULL DEFAULT '0',
`user_reg_ip` varchar(50) NOT NULL DEFAULT '',
`user_pms` smallint(5) unsigned NOT NULL DEFAULT '0',
`user_notifications` smallint(5) unsigned NOT NULL DEFAULT '0',
`user_posts` int(10) unsigned NOT NULL DEFAULT '0',
`user_newsletter` enum('0', '1') NOT NULL DEFAULT '1',
PRIMARY KEY (`user_id`),
UNIQUE KEY `user_name` (`user_name`),
KEY `user_group` (`user_group`),
KEY `user_reg_ip` (`user_reg_ip`)
) ENGINE=MyISAM DEFAULT CHARSET=utf8

O ometto il KEY (user group = gruppo utente, user reg ip = ip di registrazione utente)?

Grazie