I beleave you have 3 classes of functinality that you can implent
- first beeing that it shows that person x is playing the game, x-fire arranges stats based on that too
- secund beeing one click join. join a server by clicking on the person.
- third is in-game chatting for full-screen apps it uses overlay of some kind to display a simple xfire interface so you don't have to switch back to windows.
it seems that the xfire team is doing all the work, though. http://www.xfire.com/xf/modules.php?name=XFire&file=games
one click start and stats bit is done by the xml file where you specify the file/process I think not sure how that holds up with java running the jvm and all, you should mail them I beleave they are chatty-er then most.