I do have a question, if the outer rings and the Green Sun exist outside of sessions, and the session members can travel there and meet with eachother, granted usually only at death or great personal risk to themselves, how come they haven't come across any of the undoubtedly infinite number of other sessions' players?
Because they have no common memories that would allow them to be in the same dream bubble, and there are likely to be very few other players exploring the furthest ring. Living players are busy trying to beat the game and have no reason to venture out into the furthest ring, and dead players are unlikely to leave their dream bubbles, if they can leave at all. After all, why go out into the murky darkness of the ring when you sit around in a bubble shaped by your thoughts and memories? So, what little players would actually be out there, they would be hard to find due to the ring being infinite, and space time working chaotically out there.
As for why they haven't found any of the other sessions themselves, well they have explored very little of the ring. Only the area around the Green Sun, which appears to have no nearby sessions. Plus, Dave's map was made by taking directions from the horrorterrors, and they have no reason to send him through other sessions on his and Rose's way to the Green Sun.