An account holder can allow managers to invite and/or delete drivers to courses. Go to Profile > Account. You will see the two toggles to allow an invitation or deletion. Green means they're on.
If a manager invites drivers they come from the general pool of course licences that you have available. It's best if you have a separate conversation with the manager to clarify your expectations around this so that they don't invite people you haven't approved to have access to DT Driver Training.
Managers cannot invite other managers.