To add a guest to your account, you need to first create a shareable board. Shareable boards allow you to share projects and tasks with people outside of your company.
Once you have added a shareable board, you can add the guest by going to the shareable board you want them to see and clicking on "Guests & Subscribers."
A window will open, showing you the team subscribers on the board, and the guest list will be on the right side.
Enter the email address of the person you want to add as a guest, and they will receive an email invitation to join just that board.
Once a guest is added to one board, you can add them to other shareable boards on the account, but they will not be able to see the other shareable boards unless they are specifically invited to them.