They install and run Revel on their own Mac or device. They create an account using the same email address which you used to invite them to your own carousel. When they login, or next open Settings if already logged in, they will see your invitation pop up. If you are subscribed, they do not also have to subscribe to join you and collaborate on your carousel. In other words, your own subscription provides access for both of you or all of you to work on the carousel you're sharing.