To make this achievement easier, when you select the option to join a crew under the "Social" tab, use the search the function and type in "horde" and then hit enter, or the equivalent on a phone.
You will notice several crews with the name "Horde N Go" or something similar. Select any one of these that are available to join, and you will often get invites. You can also invite people if you have the necessary items to invite them with.
The idea of these crews is to leave the crew once you can't run hordes anymore yourself with items, or aren't getting invited by anyone; so do what you can, then leave the crew to make way for others. If you stick around doing nothing, you might get kicked.
As you join and leave these crews, you will encounter many different players, which will make this achievement unlock eventually.