At this time, the admin’s account is a part of the license and is counted as one seat. If you’d like to request this feature to be part of future development, please follow this link.
As a workaround, you can transfer your admin rights to a colleague who is actively using Grammarly. To do this, please follow the instructions available here.
Once your admin rights are transferred, the new admin can invite another person in your place. Please check this article for more information.