Have you looked through phpBBs extension database for such a referral script?
I would have to think that you'd want to have the "referrals" be registrations. If that's the case, just give everyone a link in their profile to the registration page with a unique ID (say their user ID). Then on the registration page, grab the referral ID (if any) and pre-fill a hidden field with that value. Then, when that person registers, you'll have to add 1 to points for that user (presumably stored in the user table).
You'd do the same thing with posts, for each post, you'd just increment the value of their points column in the user table.
I'm not a phpBB guru (I prefer SMF) so I can't say what tables to edit, or how to edit them, or how to deal with the files. But I'm sure there's a mod out there for this (or something very similar).
