When you register a new account on seatable.io, you are registering your own “organization” and will automatically become the “organization administrator” of your organization.
After logged in, click on your avatar and you’ll see “organization administration” in the menu:

The cloud.seatable.io is a big container of different organizations. Each organization is considered as a customer account. Inside each organization, the org-admin can add new users to the org and these users are the members of this org. Their activities are limited inside of this organization and cannot access any data from other organizations. Also, no matter if you are an org-admin or a member of the org, you cannot realize that there are other organizations: you cannot see the name or members of any other organizations.
I hope I’ve made it clear? 