You can use our custom/3rd-party Integration or Send Generated Codes feature to have us transmit the details for each order to a script at your end that would handle forum registration.
The only difference between these features is that Integration is a one-way transmission to your script (in this case your script would email forum access details to the buyer on its own), whereas Send Generated Codes receives output back from your script after we submit the order data to it, and we then include that output in the thank-you email we send to the buyer with a link to their download page.
A fairly simple approach could use the Integration method and have your script register the buyer's email address and unique Transaction ID as their forum login and password respectively. Then you could add your forum URL to your thank-you email message template and use the template codes [%payer_email%], [%txn_id%], and [%download_link%] or [%thankyou_link%] to insert that email (forum login), ID (forum password), and link to the buyer's ebook download page wherever you wish in the message -- e.g.:
Thanks for your purchase!
Please visit this page to download your ebook(s):
Your purchase includes membership in our private discussion forum, which you can access here:
Your forum login: [%payer_email%]
Your forum password: [%txn_id%]
If you'd like some help setting all this up, we can recommend the competent, E-junkie-experienced developers for hire listed at the Developer Directory link at the top of our site.