Group Index is used by game modes to determine priority when selecting objects, with lower numbers having higher priority.
Firefight Group Indexes
In the Firefight Game Mode, Group Indexes are set in AI Spawner properties, and are used to determine which "hill" AI from a spawner will go to in Firefight KotH. Nodes in the Game Mode Firefight category can use Group Index to filter spawners and units.
Firefight Subgroup Indexes
Subgroup Indexes are used to further define priority from within an Index Group.