EDIT PLEASE NOTE: AS OF 2/20/2016 THIS ACHIEVEMENT APPEARS TO POTENTIALLY BE GLITCHED FOR SOME USERS. THE GUIDE IS ACCURATE HOWEVER IT IS NOT POPPING FOR SOME PEOPLE.
Crowd events are ever occurring, yet entirely random. They can be encountered in any district at any time. Your goal os to complete 20 of them. They will appear as little red or orange dots on your minimap depending on what kind of event they are.
Examples of Crowd Events:
Protect the Citizen- They're being harassed by Blighters or Templars, kill all the Blighters/Templars before the citizen is killed.
Scare the Bullies- No sure fire way to do this, what I do is walk up to the bullies and wedge myself between them and the citizen and sneak/stand a few times till they disperse.
Tackle the Thief- You'll be required to chase after a thief, normally they're slow and awkward so you shouldn't have issue, and when you get close enough, tackle them with B.
Stealth Kill the Messanger- If someone knows how the heck to do this let me know because I've gotten all of my Crowd Events from the above three.
But those are four that show up commonly, albeit randomly, throughout the world as you're passing by. Complete them, and you'll get a notification of #/20 events completed and keep doing them till the achievement pops.