You can sign up from the Overflow website, but also if you download the app straight away.
After you download, install, and launch Overflow, you need to sign up for an Overflow free 14-day full-featured trial account, or sign-in in case you had previously signed-up. You can do so either by signing up via email, or by using the Google sign up button.
Halfway through the process you will be required to pick an Organization name. The Organization name will be used in the Overflow Cloud, where you'll be managing Overflow Documents and managing your account and subscription details.
Finally, you also have the option to add more team members at this stage, so that they get access to Overflow and start sharing and collaborating. Of course, you can always do this later from your Overflow Dashboard.