when you need lots of units very quickly at the begining of the game, just go to the same place that you unlocked the factions and set the money amount too. you can add units to your armies for free.

MAKE SURE THAT YOU SPELL THE UNITS RIGHT AND COPY THE EXACT FORMAT OF THE OTHER UNITS IN THAT ARMY.

You have now recruted the units for free but the upkeep cost will still be there.