In the Global Settings, you'll see a small but important setting titled 'Allow registrations' with an option to select either "Yes' or 'No'.
Selecting 'Yes' means that Twine becomes an open system: anybody can register an account for your system.
Selecting 'No' means that Twine becomes a closed system: users will need an invitation or to sign in using third party service (covered in the authorisation section of the Help Centre).
To keep your system secure, it's important you select the correct option. By default, Twine is set to be a closed system.