Account owners can allow managers to invite and/or delete drivers to courses. Note that managers cannot purchase extra drivers, only the account owner can do that.
Go to Profile, then scroll down. 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 drivers 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.