7th grade computer class playing doom when the teacher was gone.
FUN STUFF.
But yeah, inviting through Steam itself only works if the game was released on the platform. You can add the game to Steam so that you can take Steam screenshots, have steam notify friends what you're playing, and chat with friends using steam overlay. But you can't actually invite through steam.
Still, Steam makes setting that **** up easier. Cause you can just pm friend what sever your playing on or whatever through steam chat.
Number of times I said "Steam" in this post (not counting in this sentence): 8