I have read somewhere that the Power BI PRO user has a tenant and can manage users and services within that tenant. As a tenant, he can manage up to 25 individual free users so they can gain access to the Power BI PRO content even though they are Power BI Free users.
FYI to all, the new PowerBI Licensing has been released. The email sounds scary. I recommend going to the licensing page to get a proper understanding. There is pricing for PRO users and those responsible for sharing VS consumers.