How do I reset a guest's password to a board in my workspace? Which tells me that he doesn't remember the password.
You may suggest them following Trello docs to reset the password:
You know the email (or username) and password of your account and can log in at trello.com.You own the email address associated with your account and can recover your password at https://trello.com/forgot. NOTE: If your account was created via Google log-in, you will also be able to recover your Trello account by logging in via your Google account.
NOTE: If your account was created via Google log-in, you will also be able to recover your Trello account by logging in via your Google account.
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