I really enjoy the melee aspect of this game but I want to make sure I pick the best melee'ing race. So far I've made an Eldar and am focusing on the howling banshee, but I got devastated so far by almost every ork I encountered (though hard to tell if this says more about my lack of skill or their superiority in close quarters).

Also, the different groups within each race (chapters for space marines, clans for orks and such), do they have any differences among them? If so, how can I find out what it is?

Additionally, if I want to buy a good melee weapon for my howling banshee, is there anything good in the store? I only see two melee weapons, and one seems for the stealth class and the other for the warlock.

Noone iz betta dan ork in melee skrapin' ya grot!

I would estimate CSM being the best overall at melee.

Banshees are stuck with their starting swords for now, but you do unlock upgrades for it. And, to be fair, starting with their iconic power swords is just as badass here as it is on the tabletop. If we ever see Exarch weaponry then it's possible that we might see things like the Executioner for the banshee (nasty glaive thing).

As the green gentleman above so eloquently pointed out, I believe Orks are meant to have the edge in melee. (Which is not to say that dedicated melee specialists like banshees and assaults/raptors can't compete.)

Eldar are currently the way to go.

Warlocks are amazing when you learn how melee mechanics work. Combine that with their healing/buffs and you can slaughter stuff in melee while helping your team.

Only subfaction currently having differences in game are Space Wolves. They have Wolf Priests instead of Apothecaries (better melee since they have power mails equivalent weapons in place of chainswords, but can't equip bolters)

Orks is not realy good to start because they are bugged in melee ;(
I think scorpion are good to start !
And jetpack ork are the worst (I don't say they are weak !)

TLDR: Eldar for now expect it to shift to Orks after slated thursday patch.

Both orks and eldar are ridiculous in melee right now .... I would imagine that wont change for the orks any time soon, i hope they remove the nonstop stun locking that orks can perform as its really bad NPE and they have removed stun locking from the game like 2x now when other factions could do it. The eldar are crazy because thier mobility makes it really easy for them to engage or disengage from a fight at their whim. I'm not going to lie im actually a little worried the eldar incoming nerf hammer is going to hit them too hard .... they need to stop one shotting people unless they themselves dies from one shots .... which outside a few broken weapons RoF doesnt happen for instance if an eldar is one shotting me with a quick attack from a power sword i aught to be able to one shot them with a quick from a power fist at least maybe mauls and axes too IF im also a melee specialist .... I'de honestly prefer a world where neither of us can one shot each other but its tough to balance that when there is shooting going on too. Also if you are on the eldar team you dont have to worry about getting run over by a WS as much and due to fire dragons the enemy team will likely never have any vehicles for more than few seconds/one spawn.

basically what im saying is at present i would rate the eldar as the highest powered melee race since they have albiet slightly situational 4 classes that can one shot in melee and they are incredibly fast .. the banshee has a debuff projectile and they have SS stealth .... however i think this is going to change in the patch slated for this thursday. and it needs to .... healers shouldnt be the go to front line infantry and presently on most factions they are. between striking scorpions that have figured out a quick c toggle while fighting means you just get a free 50% permanent damage buff for slightly reduced attack speed and banshees who are still one shotting with quick attacks for some reason which may be a bug with the hit box counting any hit as a back hit im really not sure im doing the best i can to cap every time it happens and submit it ... and warlocks who can heal, one shot you in melee with quick attacks or shoot a beam that makes the person they are healing one shot you in melee and take almost no damage, as well as a ranged flyer that can one shot you with a fierce 50 lp knife attack in the back. To say nothing of the fact that many of their RANGED weapons can be inappropriately used in melee like the reaper launcher and the HM. the eldar will not only one shot you while being aggressive they can also in many cases one shot you defensively when you surprise engage them.

Orks as a race are supposed to be the best in close range and functional in midrange save a few outlyers that for game balance need to function from long range. i sincerely doubt they will retain the "heavy" trait as it is now ... the devs have on every occasion been completely unable to balance stunning and for good reason its a hard thing to balance in a game with this much fast action going on. However even if they modify/remove the heavy feature the intent would be to replace it or balance it and still leave orks being the scariest in melee. They benefit from having a very strong melee healer as far as melee DPS goes as their poison is ridiculous and they are tankier than apothecaries and have aoe poison grenade potential or healing beacon potential.

The main argument for chaos is that they can get the highest toughness rating which is a very valuable stat for melee IF you can get enough to break through the pen for midrange weapons. On the whole i would put Chaos as only slightly better than LSM for melee. However assault slam on CSM and LSM is very unrewarding and its supposed to be your initiation attack.

LSM has to be the weakest melee faction right now .... we get access to all the same melee goodies chaos gets but we have no LP efficient way to make ourselves tanky enough to be real threats in melee save the good ol shield bro but if you've played a lot of shield bro you've probably realized hes not anywhere near as powerful as the community makes him out to be he IS very powerful in his situation but since most fights over points are complete CqC bloodbaths you can't depend on a shield defending you very well when thiers people flying all around the room with quick attack and even if they dont get behind you i myself have recorded and posted situations where they kill you straight through a fully rested upraised shield because the shield doesnt even do what its supposed to always.

Couple that with the fact that our healers outside of beacons have to be in melee to heal and you start to get the picture.
We DO have some weird stuff going for us like the insane damage on Rite of Fire, or the ridiculous poison damage of the tier one narthesium i honestly am just writing these two off because i expect with any amount of sanity they will get nerfed ... since at present apoth is the most durable unit, a contender for best DD, and is a healer on top of all that. but on the whole much like chaos we don't have anywhere near as many melee weapons that are "doom in one hit" either through stunlocking or straight up super damage. We also have the disadvantage that we don't even have enough defense available to us to make it take more than 2 stray shots(pulses of damage) from ranged to kill us either ..... so its much harder to survive to get into melee on LSM as well especially since they introduced wing clipping to JPA's since even stray pistol rounds are enough to knock us out of flight and unlike the SH we have no way to engage from range effectively. Also at the ranges storm boy/JPA engage from FLAK trait allows chaos and several vehicles to not even notice we are there and gun us down because they happen to be shooting in the same general direction we come in from(yes i have had an ac shooting at someone at ground level wingclip me while im assault slamming from above).

Thanks Delgear, i totally agree with your melee review and my english is too bad to explain my point of view. Hope dev' read this

I'm not sure whether to be annoyed that you're talking about the playable factions and calling them races, or not care because this is science fantasy and the word race is invariably abused, or annoyed anyways because the typical abuse of the word race in fantasy is to conflate it with species, not faction.