While I have only tried this in Perez Park, this trick should work in other zones that have many enemies packed together.
To make this trick as effective as possible, you will need to have two different types of abilities:
1. A toggle PBAoE skill (e.g. Arctic air, Hot Feet)
2. A toggle Ranged AE skill (e.g. Storm Summoning, Enervating Field)
I'm not sure if any other archetype other than Controller has both these skills. If you only have one of these skills, perhaps you should get a friend who has the other skill to help pull.
(From Atlas Park entrance in Perez Park, Perez Park side)
Now what you do next is go very far north or very far south. Make sure that the path is mostly clear of players or this might not work. Be sure to watch out for the badguys and not get shot along the way.
For starters, maybe walk about 300 yards north then look for the nearest pack of gangsters. Toggle on your PBAoE skill (e.g. Arctic Air) then quickly hit him with your toggle Ranged AE skill (e.g. Storm Summoning). At this point, you might want to toggle on Sprint and then run back to the entrance or to your group while scooping up gangsters along the way. To scoop up gangsters, you want to toggle on your PBAoE skill and then you want to run *near* (not over) a pack of gangsters. You only need to affect one of them with the outer edge of your PBAoE in order to get the whole gang running after you. Be sure to check out platforms or parking lots for packs of gangsters that you can scoop up along the way.
When running, try to keep the gangs within visible range so that you are not likely to lose them, but at the same time, not getting shot at. If done correctly, you should have about 50+ gangsters following you.
Once you get good at this, you may want to run further down the street to pick up more gangsters. Just wait about 10 minutes for them to respawn.
The reason for using both these skills is because the toggle PBAoE is the best way to get aggro while on the move, and the toggle Ranged AE is the best way to pick up stragglers.
This should be great xp for your group or just to grief a few people hanging around at the entrance. Here's an example of it done right.
http://www.geocities.com/boogatty5/cominforyou.txt(only viewable in IE6)