You must affect a gang to one of your followers.
To do so, you have to get a recruitment order from the online market, in the pause menu. You can simply buy one or more silver war chest which costs 1,500 mirians. If you don't get a recruitment order, just buy other silver war chests until you get one. Here are the different types of recruitment orders :
- Archer recruitment
- Defenser recruitment
- Olog recruitment
- Warrior recruitment
- Savage recruitment
When done, go to the army menu, select one of your captains (in blue), select "Command" then "Training orders" and select the recruitment order in the basic perks list.
You should see the achievement pop immediately.