As you've probably noticed, all my recent lists have large units of genestealers. This is outside the norm of what we're seeing on forums and in the blogosphere, so here a breakdown of how I make it work.
***Disclaimer: I set this photo shoot up as a sample scenario, it's not from a batrep. In reality, there will be a lot more factors and a lot more movement, so take it with a grain of salt and apply it where you can in real games. Also, this was laid out on a 25% terrain coverage table with several types and levels of terrain. I assure you all models are in 2" coherency for all moves. The genestealers are a tad outside 18" to start.***
Here's a sample pitched battle deployment against CSM:
Note that when using this 20 genestealer unit (usually with toxin sacs) I almost always infiltrate them. The unit is too expensive and vital to my playstyle to risk not coming in until late game. I know some people get sqeamish about taking first turn casualties. We'll look at this closer from the chaos standpoint.
So a majority of the stealers are in cover from each firing angle, and they are in range of the tervigon for synapse and catalyst. So I give them FnP and move them up.
Still in cover...
My next turn, I shift to focus on one assault and set up the "passive assault". I have been using this tactic, but didn't see it named and broken down until Ben "Spacecurves" wrote the article for BoLS. Basically you want to assault in a way that minimizes first round casualties so your unit doesn't free itself to get shot next turn. It can also be used to pull scoring units away from objectives.
As you can see, I redirect towards a single unit, with very few genestealers that will actually attack.
So there it is. I hope this helps with in-game execution. 20 genestealers with cover and FnP are extremely resilient if they're fearless. They are a large enough unit to threaten multiple units in a MSU match up. They can often hit too hard and get shot down after they hit home, so use the passive assault to keep them locked in combat during your opponent's shooting phases.