Group Combat System

Players in the same room will join a building group automatically upon
killing a monster. Thereafter, every kill by a group member will share
experience with other group members, exactly as if the players were
fighting the same mobs.

Players must get a kill within a room's GroupTime to remain in the
group. Otherwise, they will leave the group until they make another kill.

Players that leave a room also leave the group.

Group members are denoted by green halos. Guilded group members will
have both yellow and green rings.

Enemies that join your group will remain red halo to avoid confusion
(and what are you doing building next to enemies anyway?)

Also included in this system is a DrawHPChance call for every
AdvancementCheck, fixing an oversight caused in the GainChance bar
that wasn't updating when someone else killed a mob and generated XP.