"Invited" users need to have a username. And a username is connected to a Trello account.
If you want them to do things (like adding or removing comments, labels, due dates) in a Trello board, you need to invite them.
If you only want them to be able to read, it would be enough to make a board "public".
@Pelle Chamliden You can invite anyone to join a board by email address. If they don't yet have a Trello account, they will be added as a "virtual" user. They'll get an invitation email that has a "join board" button in it. When they click that, they will be prompted to create an account. They won't be able to interact with the board until they create the account.
If you're a business class customer, you can add users as observers to your boards. They will still need to sign up for a Trello account:
If you had to thrive a new habit during a lockdown, what would it be? Trello
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