Networks is definitely not my area. But you obviously have them routed off somehow. It sounds to me like you will need to both configure their firewalls & do port forwarding for each individual computer. From my knowledge that means you would need to forward a separate port number for each computer. ie: Port 6346 for one & other numbers for each of the others. (I might be wrong here, but ... until we know more.)

Firewall instructions: http://www.limewire.com/english/content/firewalls.shtml

We really do need some more details to help you further such as what firewalls, which OS versions, what connection type, what modem-router brand name(s) & model number(s)?