Absolutely. You can add or remove users at any time. To add a user, first navigate to the Users page, click Add User, and enter the required fields. Then, navigate to the Content Subscriptions page and click Purchase a Subscription. A new subscription will become available. In the drop-down under User, select the user you wish to add.
To cancel a subscription, navigate to the Content Subscriptions page and click Cancel next to the user you wish to cancel.
Added users will be prorated from the day you add them until the next billing cycle. Cancelled users will be cancelled on the date of the next billing cycle.