Anyone connected to the network, via wireless or wired, could use ARP spoofing or DNS hijacking to perform a man-in-the-middle attack against legitimate connections over the network.
I should point out that WEP provides security only against casual attackers. Freely available tools can crack the key using deauth and ARP injection in under a minute. I would highly suggest moving to WPA2 for your network, as WPA and WEP are considered broken.
You could also consider an enterprise option such as RADIUS for network authentication, as that provides additional security against password cracking, and integration with other user management platforms (e.g. active directory) within your environment.