As I recall, Aaron was doing his own startup through YC (Infogami). Paul Graham introduced him to the Reddit guys and I believe suggested they merge Reddit and

As I recall, Aaron was doing his own startup through YC (Infogami). Paul Graham introduced him to the Reddit guys and I believe suggested they merge Reddit and Infogami. As part of that merger, Aaron gained the title "cofounder", IIRC. This is backed up by comments in that Reddit thread, FWIW.