Treasure chest can only be found in Dungeon open/instanced, and quest areas. No open areas will have them.
You may only have 1 treasure chest per floor of the area. If you are inside a party everyone may get loot unless it was a miss. The treasure chest loot follows the same loot table as the monsters and it is a fair system in which everyone has individual rolls.
Also a chest may be spawned only after a monster is killed by you or your group. One spawn of a chest counts for the whole group.
So for an example of the system...
Say a lv.51 Adept Rogue has killed a monster in the first floor he is in and in the next part of that floor he finds a chest on the first floor level of the quest or dungeon area... This is allowed because he has killed a monster on the floor he was on.
Now the lv.51 Adept Rogue finds two chest on the same floor - that isn't allowed.
Now in a party - lets say the lv.51 adept rogue and a Harvest cleric are going through. Only ONE of them can spawn a treasure chest but when it comes to loot they both have same chances of getting an item.
Note loot is not given by players - your post will be edited and if not or feel it may not then just message me.