Homefront

20/03/11 -
I attained this achievement yesterday by joining my mates, mates lobby and i can confirm you DONT need to be the host to get the achievement. As the achievement clearly states you just need to enter a game with the 16 player lobby and you should all net the achievement.

The only problem with this achievement is the servers suck at the moment so you usually get lagged out of the lobby. The server seems pretty tempermental. For example, i joined the lobby 3 times, it filled up and started but i got lagged out and others got the achievement but i didnt! i was completely gutted and started questionioning my connection, so i joined again for one last try, once the bloke started matchmaking i left xbox live party chat and pulled my mic out (to stop any lag in my connection), the game loaded and the achievement popped whilst NOT being host.

If you dont get this on your first attempt, i suggest you just keep trying or at least wait until they fix the matchmaking servers. Good Luck!

Got this achievement yesterday using this method and took less than 15 minutes!
Have someone host a Full Boat session on TA, and when the time comes for the session, start no matter how many players you have on TA(we had 3 in the session on TA). Have the host start the match, and everyone join his session, also invite anyone you know that might want to do this. We had 8 people in our lobby when we were ready to go.
THEN! Have the host send everyone a message just in case he's not in their friends list(very important, otherwise you'll have to type in names manually), and everyone except for the host agrees to scatter for 5 minutes.
For 5 minutes, everyone start joining and leaving every Homefront match you can find as quick as you can, until your recent players list is full.
At the end of 5 minutes, all the original players rejoin the Host, and then send an invite to ALL your recent players. That should result in anywhere from 40-250 players being invited.
I recommend a voice message unless you're really good at typing.. something like this: "Hi! We're doing a match for the 16 player achievement in Homefront, just a few slots left, if you want the achievement join quick please!" That worked for me :-)
You should have no trouble filling the match up if everyone did the match hopping, just wait for it.
Make sure everyone is "In Lobby" so the host can start the match, and make it GROUND CONTROL. We had 15 players unlock it first try, just one player needed a rematch to get it again, so most of us quit out and replayed. Also, expect players to go wild and start shooting enemies and each other when the match starts. Don't worry about that. Just use invites if you need a redo.

Thanks to SupernaturalH2O for the ground control idea!

This can be a very easy or a very frustrating achievement. I have now been to two groups going for it. One took a little over two hours and I think a few people may still have not gotten it. The other took about 15 minutes and everyone had it pop firste match. The key here is Ground Control. We tried several Skirmish matches and only the host would have the achievement pop. We even joined up and finished the game with all 16 of us in there and still no pop. The first match started up with Ground Control as the game type and everyone, even those not hosting got it.

Make a gaming session or invite randoms from your recent players list up to 16. As soon as they get into lobby start up a game of Ground Control and enjoy your Gamerscore.