You have to be invited to play it. The Necromancer Beta Test won't even show up in your Realm Select drop-down box if you aren't invited.
I got my Necromancer to level 70, and I tried out the sets. (They give you a vendor that gives you a set for 500 blood shards.) Since I've been using a skeleton build, I decided to try out the pet set, and my goodness... it's kind of ridiculous. The 2-piece bonus is somewhat worthless as it gives your pets a chance on hit to reduce the cooldown of Army of the Dead by 1 second. The problem is that Army of the Dead isn't that great and it's on a 120 second cooldown to start with. The 4-piece bonus gives you a stacking 1% damage reduction buff whenever your pets hit something (up to 50 stacks), which is quite a nice survival buff. The only negative is that it lasts for 5 seconds, so it will usually fall off between fights.
Now, the best part is the 6-piece bonus. The 6-piece bonus gives your skeletons and army of the dead a 200% damage bonus for every Skeletal Mage that you have summoned. Skeletal Mage costs 40 essence (out of 200), so even without any regen, you can use it a decent amount. What I did was pretty much alternate between hitting Skeletal Mage and using my essence generator (Bone Spikes), which meant that I pretty much never ran out of essence and I could easily keep up 4+ mages. I also used the Skeletal Archer rune, which gave me an attack speed bonus every time the archers (they turn into archers with the rune) hit something. Now, keep in mind that if you use the Frenzy rune with Command Skeletons, you can make them deal more damage and attack faster. I was playing on Torment II, and bosses would just melt when I combined everything. I'd usually send the Skeletons in with Command Skeletons and then start summoning a couple archers. I don't think any major boss could last more than 5 seconds or so.