Event: Biter spawned & unit groups

[BenSeidel]

I am looking for a way to control the biter group(s) created by the pollution absorption of a spawner.
Is there any way of enumerating these groups or some way to be notified when they are created / when a unit joins them.
As there are more biters than anything else in the game, looping over them to find members of groups seems excessive.

I have tried using events that are triggered when an entity is created with no success. I also can't find any that are related to unit groups, nor anything on a force that allows the enumeration of said groups.

Any help would be appreciated
Thanks!