You can connect to an external database using the wpdb class. A search for 'wordpress connect external database' will find many examples. For example, this one: http://wordpress.stackexchange.com/questions/26463/connect-forms-in-wp-to-external-database
Just be sure that you use the external database information instead of the DB_* constants you see in some of the examples:
$external_users_db = new wpdb(DB_USER, DB_PASSWORD, DB_NAME, DB_HOST);