The only real way to get this achievement would be to either have a lot of people on your friends list who have The Orange Box, or start a session here on TA. Don't make the session for the next day, make it a few days away. Due to the rarity of this achievement, set the # of players to Unlimited, and many people will join. Also, joining into one big party is a must.
Now, here are somethings you have to remember. You need to complete a full round (capture the flag 3 times, gain control/defend control points) for the achievement to pop up. It doesn't just appear. Also, make sure noone leaves early, as that can mess things up. Do all that, and it should turn out great.