Forming a crew is easy, so long as you’ve got the dough. Go to the Black Book screen to view the gangsters available at your Notoriety level. You can click their portraits to view their various stat, traits, and equipment screens.
When you have chosen a gangster, right-click on their portrait in the Black Book screen to hire them – so long as you can afford them. They will appear at your boss’s location, prepared to follow your every command. Simply click on them (or click and drag to select multiple gangsters) and start setting them loose on Chicago.