Genestealers in E41K
Genestealers have a long history in the Warhammer universe. They in fact predate Chaos in the WH40K universe.
They, at first, were thought to be native to this galaxy and were not related to the Trynid Hive Fleets. Later this was shown to be not correct, and that they were in fact part of the Trynid invasion forces, with a given cult taking over a planet several years, some times decades or centuries before the hive fleet would drop out of the Warp "for a light lunch" as it were.
Genestealers attempt to take over a planet by forming "Cults" formed of the siblings of the hybrid Genestealers. At times the hive mind is to far away to give strength to the leaders of the cults. In these cases the cults may turn to the Chaos Gods for strength.