Group Arena is used for testing your strength in team combats with other players.

All participants are grouped by attackers and defenders. Each side always has 3 players participating.

A combat is always initiated by attackers. To attack, a player needs to create a team, invite other players to join it or start a fight with random participants. A team of defenders also always consists of random players. All random participants take part in combats in absentia.
Arena is divided into divisions. A division depends on a Character’s level.

Once a day a Character is offered a limited number of attempts for engaging in group combats. Extra attempts can be purchased for coupons.
The attacking team gets a reward both for winning as well as for losing in a fight, except for those who participated in absentia. The size of the reward depends on the division and results of a combat.  

You can join the Group Arena by talking to the Warrior.