Problem: You'd like to share some data between two different drupal websites without changing any Drupal code or otherwise having to lift much of a finger. But you don't want to share information unless it meets certain requirements (for example, only users labeled "foo" in somedatabase.sometable) For our example, we'll use the users table.

Solution: Use MySQL5 Views (http://dev.mysql.com/doc/refman/5.0/en/views.html)

Our "master" users table resides in a database called "master". The database of the site that will have restricted access to our masters users table is called "banana". Assumming you're starting with a fresh instance of the Drupal schema in your database "banana", do this:

mysql> use banana;
mysql> drop table users;
mysql> CREATE VIEW users AS
mysql>  SELECT *
mysql>  FROM master.users 
mysql>  WHERE uid IN (
mysql>    SELECT uid
mysql>    FROM somedatabase.sometable
mysql>    WHERE uid = 0
mysql>     OR label = 'foo'
mysql>  ) 

Drupal will use banana.users just as it would a normal users table. No other modifications are necessary. Now only "foo" users will be included in the users table for your banana website.

Note: Drupal has a dependency that is not really documented. Every users table must have an entry that contains uid=0. It's a "stub" entry that Drupal needs to function properly. A workaround for this dependency is to include "user 0" in the results set that defines your view.

See, wasn't that easy?