You can use the simple press forum which allows you to embed the forum in one of your pages. You can then use eMember to control access to that page. This way you can make it so anyone who is logged in can view the forum.
You can also use the BBPress forum which has WordPress integration. If you use eMember with WordPress integration too then this can be a viable solution. Basically you don't let anyone register from the normal registration page of BBPress also make it so only logged in users and view the forum posts. Once a visitor makes a payment for membership using eMember, a wordpress account will be created for the visitor. He/she can then use that login details to log into the forum and view the content.