Admin users see the following message at the top of every page:
Your license restricts you to a maximum of X users. You have exceeded this limit. Contact support.
Your organisation purchased a licence for Asset Bank that restricts the number of active users. An 'active' user is defined as a user that can be found in a search in Admin->Users and has logged in at least once.
Delete users (in Admin->Users) to reduce the number of active users, or contact our support team to discuss upgrading your licence.