Then both sets of users are only ever in one board, so they are not required to be paid plan.
Only trigger a paid plan requirement when they are in two boards with you, or that group.
My advice is to possibly try and make multiple groups, up to your limit if you ever did need, and see if that bypasses, as potentiall it could, otherwise that would need to be a paid plan and may want to assign users on their next project accordingly to avoid fee.
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