There currently is not a way for System Admins to be notified of, and, approve or disapprove of a connected user in the Admin Center.
Anyone that has an email on an approved domain has the ability to create a free account, whether they were authorized to or not, and becomes counted as a connected user for 365 days after their last login.
The only way for an admin to become aware of unauthorized account creation is to review the connected users report. At that point, there is no action that can be taken to proactively manage the impact to connected user counts for an entire year.
Under some pricing structures, the number of connected users can greatly affect contract terms and cost at renewal.
Please give Smartsheet customers the ability to protect their financial commitments and budgets with visibility and ability to proactively manage connected users.