About server , I have no idea.
For clients :
Quote:


session_connect(STRING* session,STRING* hostname)
Switches to a different session on another server. The first string parameter contains the name of the new session, the second parameter gives the name, domain or IP address of the new server. This way, level or zone changing in a multiplayer environment can easily be performed.

Edition:
Pro

Speed:
Slow

Remarks:
This instruction can be used to switch the engine to client mode even if it wasn't started with the -cl command line option. Note that after switching from normal to client mode, the level has to be loaded anew. Switching the engine to client mode at run time has certain consequences for the script. IFDEF CLIENT can not be used - it's undefined because the engine was not started with the -cl option. Client dependent functions or initializations must run _after_ connecting to a session. For instance, creating a client player in the main function won't work when the engine was not in client mode when running main(). A good place for creating the client player is after reloading the level after connection.





Extensive Multiplayer tutorial:
http://mesetts.com/index.php?page=201