We know it's mostly, possibly entirely, in the United States, run by Harry and Elaine (I think Ramirez may have unofficial ties where he assists beyond his normal duties as a Warden as well). It's all the small fishes, in local communities, connected via, well, relatively low-tech communication (probably phones at best). They're quite numerous, based on comments in GS. GS also gives an important detail:
Murphy takes an active role in the Paranet.
Basically, think of it as strength-in-numbers for all those who would be too weak to defend themselves if they were alone.