Rules for playing the Genestealer Cults have been leaked and are going everywhere at the moment. So here is a few links to where you can find them.
via Xavi and Lady Atia on War of Sigmar
https://war-of-sigmar.herokuapp.com/bloggings/510
Thank you for sending these links in.
You can also see them here
http://i.4cdn.org/tg/1456686675627.jpg
The overall formation gets infiltrate and shrouded on the first turn, and can charge first turn even if they infiltrate.
Thread is here http://boards.4chan.org/tg/thread/45700794/40k-general#p45701822
Got some more coming in
http://i.4cdn.org/tg/1456688702545.jpg
We have the stats on the claws for the patriarch, it's basically if rending claws and lightning claws had a baby.
http://i.4cdn.org/tg/1456689355891.jpg
Purestrains are up a point per head but gained fleet, stealth, and hit and run.
http://i.4cdn.org/tg/1456689561546.jpg
Primus, not too impressive but not terrible either. Just has rending rule on himself so it works with his bonesword (which is as per tyranid codex). Other than that he's an okayish budget HQ with zealot.
http://i.4cdn.org/tg/1456689609405.jpg
The Magus is actually ML 2, not 1 like a lot of folks were guessing. He does have a force staff too, which is neat for tyranids. Also the rules for the familiar are there, 2 extra str 4 ap- rending attacks in addition to the usual.
http://i.4cdn.org/tg/1456689655153.jpg
Abberants look like they've got some potential, with 2 wounds each and FnP (and toughness 4, so they actually get to use it against str 6-7). The hammers seem better than the picks to me, assuming they get to pick freely in a proper codex I think we'll see a lot more with hammers. They might be a bit points costy for what they do, though.
http://i.4cdn.org/tg/1456692486768.jpg
Cultists are basically what you'd expect. The mining laser is basically a short-range lascannon, not terribly impressive but since they infiltrate I guess it'll get the job done (and at least it's also probably on a much better point budget than a lascannon as well, if/when we see a proper codex)
also here, although I am assuming that these are all coming out of links above
via Fxeni on Dakka
http://www.dakkadakka.com/dakkaforum/posts/list/1920/671862.page;jsessionid=E08391EC21D1B30A9D020883DED0E9BC#8489196