If your WordPress users are a mixture of paying customers and registrants who aren’t allowed access then a common apporach is to use a plugin such as PaidMembershipsPro. By adding code to your themes functions.php your WordPress site can send group memberships over to your Discourse site.
Then in addition to ‘Login Required’ you can secure your categories behind groups which are populated by the WordPress plugin and SSO process.
The example code is in this post:
It’s not perfect, I believe @dfriestedt was working with @justin to clean it up and make the code more widely available:
You’ve not elaborated on how you’re managing users or selling memberships on the WordPress side, so that’s the best I can do for now.