The organizational subscription currently includes annual access for 1 owner account and 3 organization member accounts (with the option to add more members). You'll be able to invite members to your organization after you've started a paid subscription. To do it, click Start your subscription on the Organizational page. From there, you will be taken to the eStore page. Fill out the information and proceed with checkout.
To add a new organization member, use the invitation link or add your domain to allow single sign-on authorization for your members. This way, new members will automatically join your organization using their work emails with the specified domain.