I have Nginx with Passenger. In nginx.conf I have line:
user pass users;
and Nginx process works on \'pass\' user, but Passenger* processe
Put the following into your nginx.conf in the http block:
passenger_default_user custom_username;
passenger_default_group custom_group;
You can find more configuration options here:
http://modrails.com/documentation/Users%20guide%20Nginx.html#PassengerDefaultUser
You can pass a user line into your nginx conf. http://wiki.nginx.org/CoreModule#user
I suggest that in the future you ask server questions on ServerFault, they usually know a lot more about nginx and passenger.
At one point the user and group were determined by the owners of the config.ru file. I'm pretty sure this is still the case.