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';
}
}