mixing durpal 6 and drupal 7

I have some drupal 6 sites sharing its users table. Now I will add a new
site using drupal 7, but users table structure (and password) changed.

Which is the best way to keep just one users table (username, uid,
password, ...) accross drupal 6 and drupal 7 sites? I was thinking of using
OpenID and configure one site just to act as a server and all the others as
clients.

Any hint is wellcome.

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