Well, there are several ways to play JK.
The easiest way to play is by GameRanger.
It supports not only to JK, but for several other games, and is free.
All it takes is a register.
The only problem with this game mode is that the number of players is not very large, and can be difficult to find people.
Another way would be by the IRC.
In gamesurge server, there is a channel where games are played online, the # jk.
You can get there using the jklauncher or another IRC client.
The difference is that the jklauncher shows you the matches that are happening.
The last way would be with the help of rbots.
In this way, if you can not play with other players you can add bots to play in all modes and maps of JK (except maps made by other players who need to manually add rbots).
This mod would be nice to make a gameplay of multiplayer maps.
These are the ways that I know and use. I hope I have helped you ^ ^