There's a few way to accomplish this but I am guessing since they will be starting threads and replying, you are referring to people that are "logged in" already. On the log in page, be sure that you set session variable with the user ID (there probably already is one).
Then you basically make the start new thread and reply buttons as triggers. When one of the triggers is pressed, you would use the DataAssist to update a single record where user Id = the session ID.
You could modify the update statement to look like this:
UPDATE points SET points = +5 WHERE userID = $sessionID";
This kind of statement would update the existing points column on the fly. Of course, you would have to add a points column on your user table. You could also create a completely separate points table and then store the user Id with each instance of points, but that would mean a great deal more work.