You can use all the silver coins you get from missions, drops, and destroying items to buy Silver War Chests, as they contain Training Orders. Don't spend any real money for these. The first Silver War Chest I opened had a Training Order for a gang of archers.
Once you have a Training Order that has "gang" in the description, go into the army menu, press
to command a follower, then
for training orders, and then select the order that has "gang" in the description and hold
to apply it.