Requesting Additional Trello Seats
If you're on an invoiced Trello plan (either Premium or Enterprise), there is a cap on the maximum number of Workspace 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 Workspace members.
If you need to add new Workspace members, you'll need to purchase additional license seats. In order to do that, contact our sales team at http://www.trello.com/contact, and they'll be glad to get that seat count increased. This process replaces the in-app purchasing process you may have experienced previously.
You can expect a response within 1 business day when contacting sales at http://www.trello.com/contact.