Hello Scott,
Thank you for your message.
Using RU Server within the local network only is actually possible - simply use the private IP address (192.168..) as the Server's address when setting up your Viewers/Hosts. You can see this case as an example in
this guide on setting up RU Server - see the text in the yellow frame marked as
"Important!".
Alternatively, you can also consider setting up your address book to upd ate automatically through a shared resource, for example Dropbox or OneDrive.
This tutorial explains how to se t up address book sync via Dropbox or a network drive in a LAN.
Hope that helps.