You don't need to make any special adjustment to auto-log-in the user in YAF forum.
Just an entry in the web.config is fine.
If you are using windows authentication, don not use the YAF's membership in web.config and the system should log-in the user to YAF too.
Let me know if you face any issue. or it would be great if you can share your web.config and i can guide you on what should be done.