Administrators can view who has an active ST Math account, and who might still have a pending invite by going to their school/district settings. Emails that don't have an active account will have a green dot to the left and show Invite Sent below, while active accounts will show the last sign-in date.
Need to resend an invite? Click on the icon to the right and select Resend Invitiation.
See also:
What are the different access levels for staff accounts?
How do I change the access level of a staff account or remove it entirely?
Auto-Rostered vs Manual Accounts