I need to remember to keep this example for some testing. This should be a good start for sharing a user and user_meta between websites. I do know that user_meta tends to have very site-centric settings at times. Original article was located at: http://wordpress.aspcode.net/view/63538464303732726666099/how-to-use-hyperdb-to-separate-and-share-a-user-dataset-between-wordpress-installs
$wpdb->add_database(array( //Connect to Users Database 'host' => DB_HOST, // I am using the same host for my two DBs 'user' => DB_USER, // I am using the same username for my two DBs 'password' => DB_PASSWORD, // I am using the same p/w for my two DBs 'name' => 'my_user_db_name', 'write' => 0, // Change to 1 if you want your slave site's the power to update user data. 'read' => 1, 'dataset' => 'user', 'timeout' => 0.2, )); $wpdb->add_database(array( // Main Database 'host' => DB_HOST, 'user' => DB_USER, 'password' => DB_PASSWORD, 'name' => DB_NAME, )); $wpdb->add_callback('user_callback'); function user_callback($query, $wpdb) { if ( $wpdb->base_prefix . 'users' == $wpdb->table || $wpdb->base_prefix . 'user_meta' == $wpdb->table) { return 'user'; } }