A party (a type of group) is a collection of Player characters that can communicate amongst each other privately (see chat), share XP, and manage the distribution of loot from any mobs party members kill.
Controls and limitations
A party can only have up to 4 characters and only 1 can be the leader. The leader cannot force members to remain in the party if he wants to leave, but can remove members.
You can identify the leader of a party by the small crown icon by their portrait.
As with any party member, the leader can also leave the party. In this case, another player will be chosen at random to become the new leader.
Party members communicate with:
Invitation, removal, leaving and promotion
Anybody can form a party (thus becoming the leader) by opening the party panal and creating a party.
There are two loot systems: free loot and shared loot.
In this system, any player can pick up anything that drops from any mob. Whoever picks up the item keeps it.
In shared loot, anyone can still pick up anything, but the items are automatically distributed in a round-robin fashion between party members.
Parties can be created from a variety of class combination.
All parties may or may not be organized/balanced. However, it is generally accepted that, to get a high rank and completing the mission quickly, you need to be in a balanced party.
For more details on party roles, see: Mission map