Metatags (Or Meta Gamertags) are used to find people that are playing similar games as you.
Here's the one I use for The Darkness.
All you need to do is setup a silver account, possibly with a name that has something to do with the game, you can get people to add the tag(you have to accept it, of course.), or get people to post their gamertags so you can add them, or add people that you find playing on the game's servers. Even if the person doesn't accept, you and the people on the friends list will be able to see what the pending players are doing.
The only issue you'll face is the fact that friends lists can only hold 100 people.