I want to make sure what happens when I add them to a card.
Am I going to be billed for them as members?
Can they not see all the cards as observers?
Observers can see all the cards on the board, not just the one they are added. The difference is that they can't add or edit anything.
Wether they are charged depends on how many boards they are a member of. If they belong to only one team board, they're free. If they are members of 2 or more, you will be billed for an extra member.
To know more about Multi-Board Guests, check this article: https://help.trello.com/article/1123-multi-board-guests
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