You do not need to order additional users in advance. If you exceed the number of users allowed in your subscription plan, during the process of adding the next user the administrator will be prompted to accept a charge for another block of 5 users. Users are added in blocks of 5 at $5 per month (annual discounts may also apply).
Some plans can upgrade to unlimited users for $50 per month. In that case, if you are already paying $45 for extra users and add another user, your user quota will be changed to unlimited. Otherwise, you will be charged for a block of 5 users the following business day after you exceed your current user limit.