If you mention a user who is not a member of the board, they will not receive a notification or gain access of any kind to your board.
Once a member has been removed from all but one board, they will change from a billable Multi-Board Guest to a free Single-Board Guest. You can remove a Multi-Board Guest like a team member on the team’s member page, under the “Guests” section.
On the team’s Members page at https://trello.com/[your-team-name-here]/members —you can also navigate to your own profile and click on your team’s name there—you can click on “Guests” to view all members that currently classify as Multi-Board Guests, with a description of how many boards they’re on. You can click on that link to remove them from all but one to convert them to a Single Board Guest.
@Julie Borders @Blair at Atlassian I just wanted to make a clarifying point in addition to Blair's response: the billing in Trello is done by Team in a Business Class account, so even if you have a team member who is not a member of any boards, they're still billable.
You can have as many non-team members as you like as members of your boards, and you won't pay for them.
If you had to thrive a new habit during a lockdown, what would it be? Trello
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