( airport join an existing wireless network in client mode)
With the 802.11n AirPort Express Base Station (AXn) configured to join an existing wireless network, it is not broadcasting a wireless network. Any issues with network client unable to "see" other clients would either be with the wireless router providing the wireless network OR that each network clients has NOT been properly configured for file sharing.
Please check out the following Apple Support articles for help with setting up File Sharing:
... and this excellent article from iFelix