For an off line network you would have to have a networking program, I'm not sure and I have my PC teacher here, but we think there may be an option (in a closed network offline) somewhere in the software provided in 'Office' to link each other together, in particular Word and Outlook. I don't have the time right now to research this, hopefuly someone around here will have an idea or two (heya M$ where are ya), but I'd google something like 'closed system messaging' "network messaging", ' computer communications', or 'offline PC networking', for a start. We are thinking that in a closed network you should be able to use Outlook to communicate through the adminstrators PC. The question arises about how many computers you are trying network as the more you have the more power you will need then it gets to a point where I think you would have to have a dedicated server to process the info. In a small network a regular PC should be able to handle this. Another consideration is if you are wireless or solidly connected too. If you're wireless you will want to set up your router's security settings to only allow access by the people in the network or you'll have a big leak happening. Our feeble stab at this. As usual I stand to be corrected and educated.
Good luck let us know if you find an answer or two as we are the curious types and when I have a bit of spare time I'll see if I can find some other answers.