How do I make changes to the user registration function? Just to clarify: I don't mean customizing the registration page. I've already managed to customize it to display what I want. What I'd like to know is if it's possible to make any changes to the function that inserts the info to the database after the user submits the form. The site I'm working on has a phpbb3 forum and I'm looking for a way to get a user registered in both after registering in the main site. I'm aware of the phpBB SSO module but that handles signing in, not registration. Also, I haven't been able to get phpBB SSO to work, either. Thanks.

I'm using phpBB3 instead of Drupal's Forum module because this is a migration from an already existing site that used phpbb. The client wants it this way and getting everything to look and function like their original site would take too long.
You can use

hook_user_insert

to add additional code to the new user account creation process.

More information on the User API