Sharing login between applications

Hi there,
I am working on a membership site that integrates WordPress and phpBB.
What I am trying to achieve is to either have the main site, WordPress and phpBB sharing the same user table OR have a unified user registration form that inserts the username and password of a new registration into the three seperate databases.

Does anybody have any advice on the bestapproachand how to achive this?

Many thanks in advance,

Simon