Given the age of the game as of the time of this post - things have slowed down on the server front and this has become much easier to potentially group boost. Additionally - I have some tips to trying it with a smaller group if you can't lock down a lobby.
1) Full lobby boost
8 people are required to lock down a lobby - have two groups of 4 search for a match of "Swarm Domination" at the same time. Hopefully you'll find the group you are coordinating with.
Once in the match - THIS IS THE MOST IMPORTANT THING - you must be part of the team that captures your closest point. So all 4 members go and stand in the closest capture point (either A or C). This is not "team" based - it is for the individual.
Once captured, you need to hold this point until the end of the match - here are some tips to make your life easier.
A) Send one or two people to site B - have one team capture it to make the points climb faster. You DO NOT have to win the match to get the achievement, so both teams will earn this.
B) Once the on screen notification "Critical Noise Level" appears - have the members at Capture Point B start shooting their weapons. Unload your ammo, run and make noise - this will trigger the zombie wave to go to point B and your A and C capture points will be largely unaffected - you should still keep 1 or 2 people at A & C, just to make sure you don't lose it to zombies.
Achievement will pop on the "Victory / Defeat" screen. If you didn't get it, my guess is that you left the initial capture too early.
2) Partial Lobby Boost:
This one is trickier. If you cannot lock down a lobby, you can still make this work, you just have to deal with the Try Hard Sweaties that still play.
I recommend having everyone in Party Chat, but queue for the Swarm Domination match solo. This will break up the group, but you can remain in communication. What is different from the Full Lobby Boost - well, holding down the capture point becomes more difficult because you've got people in addition to zombies to deal with.
The solo queuing helps because you can do call outs when the Try Hards are mobilizing on your friends' capture point. A little heads up is always nice. You still have to defend, so it is best to have two people guarding the location at least. I used the Trapper class because they have Claymores that help protect some of the entrances.
The Critical Noise Level thing is still super important - make as much noise as possible at the neutral site to lure zombie waves - this may not work with Try Hards drawing them to your point.
If a zombie wave is coming to your spot - good luck - you can fight them off by staying off the main stream and picking up anyone that comes at you, but keep an eye on the location - you can lose it to zombies if you get killed or abandon it for too long. If you do die/leave - you can hustle back to get it before it goes neutral, but the window is relatively slim.
Achievement will pop on the "Victory / Defeat" screen.