If you're on an invoiced Trello Enterprise plan, there is a cap on the maximum number of billable members you can add, set to the number of seats that you've purchased. If you reach that cap, you'll temporarily be unable to add additional billable members. If you need to add new billable members, you'll need to click on the "Add more seats" button available on your Enterprise admin dashboard.
This will redirect you to a page where you'll be able to purchase additional seats.
Your seat count will automatically increase once your request is complete.
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