Once the licenses are purchased, user can manage all subscriptions, for example, adding more users to the plan, cancelling subscription, by using "Subscriptions" menu from the left hand side bar as shown in the figure below:

User will be taken to the Subscription area. From the table, one can click on the pencil icon to edit any particular subscription plan. 

On clicking the pencil icon, user will be shown Manage plans screen. This will allow to add/remove users from the existing subscription plan. 

Any new plans can be purchased by clicking on PURCHASE button shown under a particular plan.