Hover over the right-hand side of the board menu.
Click on the options menu (…) and select “Share board”.
Turn the switch on which says “Publish to web and share this board with anyone”.
Copy the given link.
You can now share the copied link with others. (Anyone with the link will be able to view the board).
Click on “Regenerate Token” if you want to invalidate all the previously shared links. A message asking “This will invalidate previously shared links. Continue?” will pop up. Click “OK” and the token will be regenerated.