Ok here's what I just did to get the achievement. Your gonna need mabye 20 free places on your friends list. Jump on tf2 nice and early in the evening and start playing games in big servers with many players. This way you build up a long list in the "players" section of the xbox guide. Then go through this list and fire friend requests at everyone still playing the orange box. Once you have 16-20 confirmed friends on your list playing the orange box, start a 16 player game and party and fire invites at all of them in quick sucession. With about 20 players to invite you statistically need less than half to accept to get this acheivement. If it doesent work first time keep using this technique and you'll get lucky soon. Also if you find yourself stuck with only 6 peeps on and need one more or whatever, keep the game going and wait for randoms to join the server and then fire a FR at them, you'll eventually get lucky. Good luck!