You may want to turn of/lower your firewall, it may prevent you from joining games or for people joining yours. If you are not able to join any games try Port Forwarding. XBC uses Port 8602 on UDP and TCP. I am not too good in that area, but I can try to help you. Go here-->http://www.portforward.com/help/pfprogression.htm for help on Port Forwarding.

This was taken from my thread- http://halo3forum.com/showthread.php?t=73677
I hope this helps

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