define('CUSTOM_USER_TABLE','wp_users');
define('CUSTOM_USER_META_TABLE', 'wp_usermeta');

paste it in wp-config.php and make sure the prefixes are right (wp_* is default) 
also the blogs must be installed on the same database with different prefixes.

For example, you might have a primary blog with the table prefix wp_ on the database wordpress_blog and you install a new blog on the same wordpress_blog database but with a different prefix (this is important) - you can share the the primary blogs already populated user tables (in other words a shared login).

Shared login in Wordpress

database, wordpress

PHP