User blog comment:MarkvA/New details on "Land Grab" multiplayer/@comment-97.103.169.174-20101030223130/@comment-1265875-20101104195616

In the description for the achievement it says 'unique players' so yes you must kill 8 DIFFERENT players one time each.

The easiest way to get it is with 8 other friends of course:

1. Start a private Free Roam. 2. Invite 8 other friends. 3. Make sure no one's in a posse after everyone gets into the room. 4. Have a single person touch the Land Grab post. 5. The other 8 should get together for a 'group hug' somewhat near the post. 6. Whoever has the post activated tosses a stick of dynamite into the crowd. 7. Achievement acquired! 8. Move on to the next individual who does not have the achievement and repeat from step 4.

This is actually a little more difficult than it sounds because trying to get 9 gunslingers to cooperate is a bit of a pain in the arse. :D