Again, this can be done in two ways:
1. Proceed as described above (create your own team and invite friends).
2. Proceed as described above and join the "x360a" team.
Either way, as soon as there are 15 people on the team, go to the dressing room and the achievement will unlock. You don't have to play any games to unlock this achievement. Please note, You only needs 15 people on the team they don't actually have to be online or in the lobby/dressing room.
The server for this game is now closed and this achievement is no longer obtainable.