Daimler is the manufacturer, through their smart division. Penske signed an agreement with them to be their sole distributor in the US. That makes PAG responsible for setting up the dealer network (though they don't own most of the dealerships), marketing the vehicle, setting MSRPs, etc.
This approach is a little unusual. Most large manufacturers have their own subsidiaries to take care of this stuff. For example Mercedes-Benz Canada is the smart distributor in Canada. For the US, Daimler decided that signing an agreement with a third party (Penske) made more sense than going with Mercedes-Benz, USA.