Genestealer Cults are literally their own faction. This we knew from early rumored bits, but now we know they exactly the same way as Tryanids do. So I presume they are able to ally with not only themselves, but Tyranids as well as battlebrothers.
You can see the leaked images here.
https://war-of-sigmar.herokuapp.com/bloggings/515
You can see the full rules for the all the Genestealer Cult units here
http://natfka.blogspot.com/2016/02/genestealer-cult-40k-rules.html