Seems the Shadow Warrior multiplayer goes through LAN, so you need to use something like Hamachi. Unless everybody is a real tech-savvy, it will be difficult to set it up.
Also, gonna re-suggest Quake Live:
http://www.qlprism.us/ . Free, fun and that old school feel

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Since Worms came up, I would suggest Liero, unfortunately the community is pretty much DEAD, so I'll suggest King Arthur's Gold. A mix between Terraria and Soldat(which is Worms in realtime pretty much):
http://kag2d.com/en/I got CS:GO, but GC: Imposters is fun too!