When you create an account with Wet Ink, you'll create an organization. Each organization will have it's own subscription. Wet Ink offers a free subscription as well as many other pricing options.
Under each organization, you can create as many communities as you would like and run as many classes as you would like (depending on the plan that you have chosen).
Students and instructors may be part of multiple organizations. They can access these organizations through the 'Memberships' page.