phpbb Tik

Hello,

I have an existing Drupal5 site with 10 000 users, and an existing Phpbb2 forum with 5 000 users.
There is a partial overlap between the site and the forum: I guesstimate that at least 3 000 users are in both databases, but not always with the same username, password, email and certainly not with the same id.

I would like to integrate both websites, find some way to make all site users also forum users and vice versa.
But I wouldn't even know where to start.

My case is different from most cases I read about, because I won't be starting from scratch.

Any ideas?