"Join Error" problem

[Robotic Pope]

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.

[Raw Kryptonite]

I'd suggest getting with Intifadah to see if you can join on each other.
If the majority of rooms at this point are in the US, then the game may not let you connect due to latency issues. Playing over long distances can cause network errors or issues.