If you are in a premium workspace, you can add guests to just one board, they are called Single-board guests
If you add them to more than 1 board, they will become multi-board guests and will be billable at the same rate as workspace members.
Trello will give you a warning if you try to add someone to more than 1 board, that you will be charged.
https://support.atlassian.com/trello/docs/board-guests/#Guest-types-and-cost
Of course, if you are using a Free Trello workspace, ignore all of the above, everyone is free in that workspace and can do all of the things you mentioned...
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