Not sure I understand.. 'Auto subscribe to topics I post in' means as it states, the user has to post to a topic to be subscribed to it. Once a user has posted in say 'Topic A', you would then view the next tab of the subscriptions profile page 'Topic Subscriptions' to view the topics the user is subscribed to.
This is different to Membership Subscribe. For that to work firstly the chosen usergroup must have the permission enabled to subscribe to forums assigned to the usergroup and forum in question. Also, you only get the option of auto subscribing a usergroup to a forum when you add a permission, not when you manage one currently in place.
Hope that helps..