At the time of the release of this game, it was pretty easy to get this achievement. You just joined an online match and most of the time there where sixteen people playing the Capital Ship game. Achievement unlocked!
However, in the present time, it is much harder. The online multiplayer is completely dead and even boosters are hard to find.
There is a best way to get sixteen people is this: to create a gaming session, add this game to your boost list, send some people on the boost list a message or try to go to other big achievement related site. Alternatively, you can look up the gaming schedule on the calender and see if you can find someone's session, but it's very hard, though, you may get a chance.
When you finally have a group of sixteen people, then I suggest the host starts a flag game and invite everybody. When the game gets 16 people in a match, the achievement "TCS Tiger's Claw" pops up for everyone (including you).