If you want to chat with some people, there have to be a protocol which is used. Kopete understands various protocols. For some protocols, like IRC and Jabber, you can setup your own server. If you have your own IRC-server, you can enter it yourself, and you can somebody else let enter it, and then, you can talk with eachother.
With Jabber, it is something the same like that. You setup your own Jabber-server, let somebody register (only once), and then he can talk to you after he added you.
These options works the best if you have statics IP's. Dynamic IP's is not a big problem for IRC (except you have to change IP's first), but for Jabber, you need some extra tricks.
Of course: you can setup you firewall so that only connections from a specified IP are accepted to minimise the risk of having a server.