The licence is per user, you either have 2 workspace users or a multi-board guest, both are chargeable. Go to the Members section to see who is a workspace member or multi-board guest:
https://support.atlassian.com/trello/docs/board-guests/#Why-are-some-guests-billable
You must be a registered user to add a comment. If you've already registered, sign in. Otherwise, register and sign in.