Requesting Additional Trello Seats
If you're on an invoiced Trello plan (either Business Class or Enterprise), there is a cap on the maximum number of team 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 team members.
If you need to add new team members, you'll need to purchase additional license seats. In order to do that, contact email@example.com, 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 firstname.lastname@example.org.