hi Gregger
in my experience nobody reads T&Cs when joining forums b/c they are all the same 99.9% of the time… so you can slip something in the end which permits you to email them with “related offers”
Obv if someone unsubscribes you can add them to the unsubscribed group in your forum software via backend (I use VBulletin for mine and it’s easy to manage, maintain and bulk-email members or groups)