Multisite platform with shared users

Hi,
Does anyone out there have any recommendations on how to best setup a shared user base on a multisite platform with over 20 sites?

We have about 20 or more sites running on a Drupal7 multisite setup. We don't really want to mix our content editors and external users in the same table. Each site has its own database and its own user table with the content editors. We wish to setup an external users table which will store all our external users that we can share across all our sites.

Anyone have any good ideas or recommendations on how to best achieve this? Or anyone come across this problem before?

Thanks.