Register a demo account
A personal demo account is not required to use the Booksonic demo server, if you want you can just sign in with username and password demo.
That said, if you plan on actually finishing the books on the server it can be a bit annoying sharing an account with others since it means your bookmarks will be reset from time to time.
To avoid sharing an account you can sign up for a personal demo account below. Once you have an account you simply go to https://demo.booksonic.org in your browser or set it as the adress in your app.