If someone is already subscribed to my newsletter, but they want to be added to a specific waiting list for a future course through a new landing page, I'm assuming that if I put a "waitlist" tag on that landing page, it will just add the tag to their email? Or add their email to that tag (I'm not sure how it works exactly). My main questions is just if someone is already subscribed, how to then allow them to join a specific waiting list.