To start using Overflow, you can sign up for an Overflow account from the website, download the app, and start your 30-day full-featured trial or pay for a subscription straight away from the Pricing page and then download the app.
If you're using Overflow by yourself, then you'll appear to be the only member of your own organization. You are the owner of the organization. You will be sharing under this organization.
If you later decide so, you can add more people to the organization for collaboration purposes.