I'm not sure what you mean by "database file" but the easiest way to manage users and groups of users is using the web interface. Start the server and go to
127.0.0.1:7251 (provided you didn't change the port) using your web browser to access the web interface.
Port forwarding is going to be required if you're behind a router. How to setup port forwarding can differ greatly depending on the type of router you are using. Usually you login to your router and configure it using your web browser. The exact address can differ from router to router.
192.168.1.1 is a common address for Linksys routers.
By default, the ports you want to forward are 7250 for TCP and UDP. To find the address you want to forward the ports to, run
ifconfig from the command line and look for your current address. Usually it'll be labeled next to eth0 for a hard line connection.
I'm sorry my explanation is a little vague. If you need further help with port forwarding, please post your router's model number and I'll hunt down more specific details.
On a side note, I would like to add that even though Imptalk supports peer-to-peer communication, not all routers do. Clients who fail to establish a peer-to-peer connection will automatically fall back to a client-server connection.