Multiplayer
Forming a group with other players gives you numerous benefits. You can easily communicate with your groupmates using the Group Chat command (default: "g"). You can see the amount of damage your group members have taken, and you share rewards from missions. There are also certain missions that are only available to players who are grouped together.
/group create forms a new group.
/group invite invites a player to your group. Only the group leader can invite people.
/group join
/group leave returns you to solo status. If you were the group leader, a new leader is appointed.
/group view lists all members of your group.
Also note, if you log off, you will automatically be removed from your group, and you'll need to be re-invited if you wish to rejoin.