If the only users on your website are people who have paid for the product, and you only have one product (one level of user), then you should be able to use Single Sign On with the wp-discourse plugin to do what you want.
If you have some users on your website who should be able to access the forum, and other users who should not be able to access the forum, or if you would like to give different levels of users access to different parts of your forum, then the way Single Sign On is currently set up with the wp-discourse plugin isn’t going to work for you. I’m working on a plugin that will extend the wp-discourse plugin to make it possible to control access to a forum based on membership in a group.
Check out the topic mentioned by @anon29908737 for more details.