Cheapy, having set my first beacon I think I can explain what it is and how it works.
Now, if you want to play a certain game with friends, you either invite someone who is already playing the game, or send an invite to someone else online who may or may not want to play.
I set a beacon for Gears 3 and commented ( defaults to I want to play this game with friends) that I want to play Horde with friends. No one was online at the time, so I started playing MW3. A friend popped on line - xxsmokesmadbluntsxx is online - as it usually displays. A little later, a notification popped saying xxsmokesmadbluntsxx is playing Gears, and it had the little sunshine icon for the beacon beside it.
It seems like a great way to hook up in a specific game without bugging your friends with blanket invites. More like "hey, I want to play this sometime, doesn't have to be this moment, just whenever." or you could say in your comment "How about a Gears Horde run on Friday at 8"