To start using Overflow, you can just download the app from the website and start your 30-day full-featured trial, or pay for a subscription straight away from the Pricing page and then download the app. In both cases, you'll be creating an Overflow account during the process.
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.