The elaborate alternate reality game that served as a viral marketing device for Portal 2's release was not entirely devised by Valve. In fact, the developers of the 13 games of the "Potato Sack," the group of indie games central to the ARG's final resolution, say they were involved in the scheme's design from the beginning, with quite a bit of creative freedom over how it all played out.
Edge Magazine, reaching out to the studios involved, reports that they were brought in to Valve headquarters in December for a "Cross Game Designer Event." There, Valve chief Gabe Newell gave the studios "free rein" to design the ARG, and they "were also given access to any IP we wanted," Dylan Fitterer, creator of Potato Sack game Audiosurf, told Edge.
"Valve gave us a framework and said: ‘You know all about GLaDOS. Now, she wants us to break out into the world. Go make it happen.'" said Ichiro Lambe, president of developer Dejobaan, which had two games featured in the Potato Sack.
Fitterer said the "bump" in publicity and gameplay (if not also sales) for Audiosurf translated to 6,000 people playing it, simultaneously, as gamers strove to unlock Portal 2 early by playing Potato Sack games, according to the continuity of the ARG. Audiosurf, released about three years ago, had about 300 simultaneously players at any given moment in the run-up to the final weekend.
Edge says a comprehensive feature breaking down the creation and execution of the Portal 2 ARG will be published soon.