If you are using a router it might be blocking some ports needed to connect to other players. I don't know which ports the ps3 uses but you need to find out and make sure they are not getting blocked by the router. It wouldn't affect you as a host as you would not know that other people are getting a "join error" when trying to join you.
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XBL GT: - Robotic Pope
HyperLobby CS: - Robot_Pope
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