By default, if you are the Administrator of your organization, you are already a member of the organization. This means you can invite two other people to your organization for free. The main benefits of adding users to an organization are the ability to automatically back up and sync members' data via the AeroFS Team Server and manage users via your Web Admin Panel. If you want to invite a fourth user to your organization, you will need to upgrade to the Professional Hybrid Cloud Plan and the cost will be $40/month and every additional user will be an extra $10/month, as shown in the table below.
This should not to be confused with sharing files/folders with other users. There are no limits to how many users you can invite to a shared folder under the Personal or Professional Plan. In other words you can share files/folders with an unlimited number of people outside your AeroFS Organization for free - the caveat here is that their data will not sync to any of your Team Server instances and you will not be able to manage them via your Web Admin Panel.
Number of Users in Organization