To be able to recruit 5 gang members to follow you around, you need the Leadership III skill. For this, you need to reach Level 5.
Level 2 - Leadership I (unlocks automatically)
Level 3 - Leadership II (unlocks automatically)
Level 5 - Leadership III (unlocks automatically)
It takes 1000 XP to gain 1 skill point. It takes increasing amounts of spent skill points to gain levels.
After level 5, walk around and find allies to recruit. Your own territory will have more allies walking around than enemy territory. They wear green uniforms and show as green dots on map/minimap, and green highlights during Eagle Vision. Simply press
when you're close enough to 5 different allies to make them follow you. Once you have 5 followers at the same time, achievement!