If you need to offer additional content to users that have subscribed to your newsletter then it's easy when you're using Campaign Montior.
If the additional content is in the chunk subscriberContent then you can use the following to display it to users that are subscribed to your newsletter. While if the users are not subscribed then they are shown the newsletter subscription form.
[[!campaignmonitor.ifSubscribed &email=`[[!members.useremail]]` &listId=`the list id` &then=`[[$subscriberContent]]` &else=`[[!campaignmonitor.subscribe &listId=`the list id`]]`]]
You need to replace `the list id` with the ID of the mailing list that you're checking against.
Note: Users do have to be logged in for the above to work as their email address is required.