It's at the Cantina where you are able to purchase the rights to hire mercenaries. They are then hired during a mission at the Merc Compound.
War Pigs - 4 Elite Marines
Devil Dogs - 2 Elite Firebats
Hammer Securities - 2 Elite Marauders
Spartan Company - 2 Elite Goliaths
Siege Breakers - 2 Elite Siege Tanks
Hel's Angels - 3 Elite Vikings