First thing you're gonna wanna do is download GameRanger[www.gameranger.com], set it up (This will require adobe flash player), and make an account. Next just scroll down to the BattleFront II servers and pick one and join. Hoorah you're now playing online with people. (This is what I use. Personnally after trying Gamemaster I can safely say I hate it.)

Alternatively you can use GameMaster[info.swbfspy.com] which makes the ingame server list work again.
Just go the site and follow their directions. You can then Launch the game from the new BattlefrontII.exe or from Steam. Once in the games Main Menu don't bother with the Muliplayer button, Instead click the Join button and it will bring up the server list. Using this method allows you to not have to use an actual third party program which is what Gameranger is, BUT I just tested it and I only see maybe 4-6 servers with people in them and none appeared to be very full. I recommend just trying both methods if you like, but I only use GameRanger and have been very pleased with it. :D

IMPORTANT!!!!
Please make sure you backup your original BattleFrontII.EXE or just change it to BattleFrontII_Orig.EXE like Gamemaster tells you too because using Gamemasters .exe stops Gameranger from working.

This is a problem I ran into after purchasing the game through Steam. If you got it through Steam like me then the fix is extremely easy. Go to your games list and right click BattleFront II, then click show my CD key. Keep this little window open so you can refer back to it. Now navigate to your games directory which normally is Programfiles (86)/Steam/steamapps/common/Star Wars BattleFront II/ Install/ Now open the CDKey.exe and put your cd key into this window. BOOM. You should now be able to join the server you previously couldn't. :D

GOG VERSION: -Credit- Vaalc4r
I take no credit for this but a simple Google would point you here.
GoG Version fix[www.gog.com]

Hope this helps people. I know it helped me. This game is awesome!!

This guide is a WIP and I will add more later when I have the time and do more research to help anyone that runs into more problems. I used to play this game on my PS2 and loved it and allthough I did play it on my PC years ago as well.. I Pirated it and hated myself for it because the Devs STILL deserve money for this fantastic game. So I bought it today and have enjoyed the small ammount of time I have had with it already. If you have any questions please comment and I will do my best to help. Also feel free to rate this however you like whether it helped you or not. :D.