An account holder can allow give managers permissions such as inviting and deleting drivers, changing groups and adding or editing other managers. Go to Profile > Account. You will see the toggles. 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.
When changing groups, remember that this can remove access to courses from some drivers.