The guest user functions similarly to a user in a classic ticket system. Initially, he has no access to content and cannot view anything.
On Boards:
Once a guest user is added to a board, he can see either all cards or only the cards assigned to him, depending on the configuration. Editing cards is not permitted. However, if allowed, he can add comments to the cards.
Several elements within cards can be hidden from guest users for privacy and security purposes, including specific attachments, task lists, and custom fields. Additionally, guests cannot see other board members unless guest visibility is explicitly enabled per board – in which case they can only see other guest users.
In the Inbox (new in Pro version):
With appropriate permissions, the guest user can create cards directly in the inbox. Once a card is moved from the inbox to a column, the guest user can only use the comment function for that card – provided this is enabled.