Pavel,
What I did was upon process of payment, the following occurs:
Take the first part of the email address, ie pavel@site.com would become pavel.
Repeat these steps for both the member area (data stored in ListMail DB) and the forum (phpBB) databases.
1. Search to see if the username is available. If not, increase it by 1, ie pavel1 and check if that is available.
2. Generate a password.
3. Enter the user into the database.
The member area is set up with a cookie-based authentication process validating against custom fields in the LM database.
To allow people to change their forum password via the member area I had to modify the forum code to remove md5() encoding of passwords when storing and checking. (all md5 does is prevent hackers who have already gotten into your database from knowing people's passwords).
Other slight modification to phpBB to allow for "quick entry" of username and password via a link. I also removed the "register.php" script so people cannot sign up and further restricted potential signups by making every "registering" user have to be validated by the admin (though, they can't register in the first place..)
I have offered this information as a step in the right direction for you. I cannot provide the exact method or do this for you, at least not cheaply. :wink:
Good luck!