I have just been charged for 3 users - wondering why my guest on one board only has been billed for as a full user? I was not made aware of this in any form when adding the guest of any charge. If a guest is truly charged the same, then why on earth bother with the guest option in the first place? I am happy to pay a portion of the user fee, but not a full user charge - that is not right.
For Business Class, a guest is billable if they have access to multiple boards. I know you mentioned they have access to just one board - do you know if a Team Admin could have approved their access to another?
See the How Billing Works page, under "What constitutes a billable member?"
Single-Board Guests - If you invite someone from outside your Trello team (this is called a Guest) to your board, they're free on their first board. Once the Guest has been added to a second board, a team admin will need to approve, as they will become a billable Multi-Board Guest. However, once that has been completed, any member can add that Guest to subsequent boards.
Also see this page for Multi-Board Guests
If this user is a single-board guest as far as you're aware, I would suggest contacting Trello Support for assistance :)
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