If you want just Orcs then i think the chosen of dreanor are also recruiting green orcs.
if you want grunts (but there are not only Orcs) there is the Thirteenth (hand of Conquest)
those are the two that i know of, i hope this was helpfull. Dont worry if Orgrimmar itself looks empty, most ot the Horde guilds are somewhere outside in Azeroth most of the time.