Ideally, this information could be shown in-game, maybe with a one-liner or tooltip on the profiles menu, right where the user name and password are to be entered. Another tab in the help menu could be the right place for some further info, such as the purpose of those player accounts (experimental lobby and voting for add-ons?).
Thanks for reading, and keep up the splendid work

EDIT: To register a new account, either go to the login menu or to the players tab of the options menu. Then hit the plus icon to add a new user profile and enter the necessary information to get an online account. If you have previously played with an offline account, don't worry, you can login with any of your local profiles.