The player panel allows you to define how you interact with other players in a multi player game.
At the top of the list of players there are five buttons.
The left most button allows you to group with the player selected in the list.
The second button allows you to be hostile or friendly toward the player selected in the list.
The third button allows the selected player to loot your corpse when you die.
The fourth button mutes messages from the player selected in the list.
The right most button mutes messages you send for the player selected in the list.