Hi @[deleted] , welcome to the community!
Yes, if you invite your colleague to just one board under your Workspace, this is not considered a billed member. The same happens for your colleague, if they invite you to only one board under their Workspace, it will not be charged. Billed members are only members of the Workspace and multi-board guests, which are users that have been invited for more than just one board.
In case you have any queries about it, please take a look at our full document explaining what is a multi-board guest - https://support.atlassian.com/trello/docs/multi-board-guests/
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