To be able to view/access other computers on a network they all need to be set-up into the same Workgroup
I have no idea how Digital Dining POS works but if you have a broadband connection installed and run a wireless access point in the banquet rooms then anyone using this connection will not be able to access any other computers using the same connection without first being in a workgroup with them. For them to be in a workgroup with your machines you would have to set it up and give the details out. If you don't do this then you have nothing to worry about.
Although i do have experience in dealing with Credit/Debit Card transactions and i must say we have a separate dial-up connection for the PDQ Machines to connect to the banking systems. --> This ensures that details cannot be copied over any network as the machine is on it's own connection and not part of a network.
Hope this helps in some way.
~ E-Mu ~
"Emu, You Moo, We All Moo for Emu!" <-- Thanks to Animal
"If at first you don't succeed; call it version 1.0"