It is a bit difficult to tell. In the actual chapters, they named them "Legion of Torment", "Legion of <insert some menacing name here>", "Burning Legion", and other broad names. They were never really specific about it.
However, the map of those chapters can lead us to a decent guess. In The Burning Crusade, Illidan uses Magtheridon's blood to forge a new army of Fel Horde. It is reasonable to infer that the orcs who helped him are the same ones involved in Illidan's plight. So I went ahead and looked at the different clans in Hellfire Citadel.
Now, taken from an excerpt from "Laughing Skull Clan" (one of the clans in Hellfire Citadel) on wowwiki:
Laughing Skull Clan said:
Following the destruction of Draenor, a brutal pit lord named Magtheridon rallied the surviving orcs under his rule. After Illidan captured Black Temple and imprisoned Magtheridon, the forces of Outland (as named by Magtheridon) were once again united under one banner. The present-day Laughing Skull clan is now based in the northernmost reaches of Nagrand, possibly still under the sway of the aforementioned pit lord.
http://www.wowwiki.com/Laughing_Skull_clan
Which explains why throughout Hellfire Citadel, we see Shattered Hand, Laughing Skull, Shadowmoon, Bleeding Hollow, Bonechewer, etc. Now, Shattered Hand are dominant in Shattered Halls (the last heroic 5-man instance of Hellfire Citadel), and Kargath Bladefist is the "self-proclaimed warchief" of the Fel Orcs. So you can definitely say they played a hand in the grand scheme of things.
As for the ones in Black Citadel--if WoW is anything to go by--the only types in Shadowmoon Valley/Black Citadel are the Bonechewer and Shadowmoon fel orcs (in the actual TBC game).
Shadowmoon Clan said:
However, the remainder of the Shadowmoon clan somehow managed to survive the destruction of Draenor, and became fel orcs in the service of Magtheridon. When Magtheridon was toppled by Illidan Stormrage during the siege of the Black Temple, the Shadowmoons became part of his armies.
http://www.wowwiki.com/Shadowmoon_clan
In those chapters, there might've been combinations since there were multiple orc teams. I'd choose between Shadowmoon or Shattered Hand if you had to choose one.
