Go to your main menu, select "Konquer" then "Towers Of Time". Effectively complete the 4 tutorial guide towers to unlock the main towers. These is diffrent every couple of days. Among them is normally a boss tower. Since it just has one challenger in it, you can inform. Select the boss tower, and it will show the "Group Battle" screen. Select "Search For Players" or "Invite Players". Once the match starts, you will unlock the "Teamwork" trophy.