Party

In contrast to a guild, a party (パーティー) is a temporary group formed by players with a common mission or goal, such as completing a specific quest. Although some parties disband immediately after the goal is achieved, others -- such as the legendary Debauchery Tea Party -- are long-lasting.

Benefits
One of the main benefits of forming a party is that every player within the party can see the HP and other stats of other party members on his screen. They are also able to easily keep track of the position and distance of other party members when they're in the same zone.