The Crew

The achievement description is misleading.
I got the achievement by the time i got the ingame award (press the left stick while watching the map und press RB until you see your awards - you can see your progress there) for 5000 players.

To clear things up:
Ok, i try to explain (my game is German, so I don't know the exact names for everything):
Go to the tab at the bottom (something like 'misc'). Then browse to 'crew' and then choose 'cooperative'.
The last icon in the second row is 'meeting new people'.
Here you can check your progress towards the achievement with the same name ;)

I got the achievement while doing a mission. Hence i think you must not actually see the other players in the game but they must be in the same session.

To get this achievement you must see 2,000 different gamer tags pop up on your screen that are in your area of playing.

You can check the progress towards the achievement in the Map Menu- push the left thumbstick down to bring up the tabs menu- Go to awards and down to Crew- select this with A and go to Cooperation- Here go to the one with the Question mark. It'll track the percentage of people you've met. To get how many you have and need to just multiply the 2,000 times the percentage. The equation will be 2,000 x .56 (56%) = total you've collected. Fast way to get this is going to city to city and drive from top to bottom to discover new players in that area. Each percentage is 20 players you've discovered. The TA stats for this came are not correct, it'll say you've seen 4 to 5k people when you haven't so don't go by that.

You can track this achievement by going onto Awards > Crew > Cooperation > 8th icon (its a question mark logo)

Anyone looking to boost this, we've been doing it at the Square shaped orange stunt park top right just south-east of Chicago. You can simply idle or rubber band your controller to dive into the water. The more people who show up each day, the faster it will go.

Big groups here constantly holding RT driving into river and respawning.

Boostable method:

There is a somewhat boostable method to this, and there doesn't seem to be anyone that's posted about it so here it goes, you can gain 2-4% per hour using the method below:

Simply put, you gain percentage towards this every time you load into the game. This can be abused easily by constantly re-loading into the game while at the Stunt Park, which is located somewhere between Detroit and Chicago. (At the very southern point of what should be Lake Michigan). It's orange and looks like a skatepark with ramps and loops along the waterfront. There are usually a lot of people AFK here at this location, which makes it the perfect place for this method.

Now if you keep doing this repeatedly, you'll notice that not all of the time you'll be in the game with the same people. I've heard this be referred to as "being in a different instance" in the game as somebody else. So your mileage may vary every time you reboot the game. Sometimes you'll get +5 or whatever the amount of people around you is when you load back up, and sometimes there will be 0 people around you. Just restart, you'll get people more times than not.

One Step Further:

Now you want to take this method a step further you say? Yeah. I got you. If you can double box, then this will be perfect. Otherwise, a friend who is about to go AFK is your other option.

1) Have your alt/friend go to the Stunt Park.
2) Make sure the instance your alt/friend is in has a lot of people around them.
3) Have your alt/friend invite you to their Crew.
4) Accept Crew invite on your main.
5) Now your alt or friend can AFK at the Stunt Park.
6) Fast travel to them if you're not already at the Stunt Park as well
7) Repeat the rebooting method!

Now take note, even after setting up the "One Step Further" method, you can still load into the game and not have anyone around you even though you and your alt are in the same location. "Being in the same instance" is still in play here. This method just increases your chances of loading up into the same instance as your alt/friend that's around the most people in one instance.

AFK Method:

1) Grab a wired controller
2) Fast travel to Stunt Park
3) Face the water
4) Tie rubber band to your right trigger
5) ???
6) Profit!!

I hope this helps you server closure chasers. This method should be useful to those who have beaten the game and are just AFK'ing for this achievement.