Fallout: New Vegas

this mission and achievement have been updated and are no longer glitchy as long as you have title update 4 (the games most recent update)

to start the mission talk to tommy torini in the tops casino is hi in the room at the top of the stairs as soon as you go in to the casino (if you have a high enough barter you can get 5% extra caps for each person you recruit)

now you can only recruit the people one at a time becasue every time you recruit one you have to go back to Tommy to get your reward then you have to go and get the next one

here is the location of each person you need to recruit:

Billy Knight: outside the tops casino next to a gate

Hadrian: Inside the Atomic Wrangler. You will have to convince james garret to let him go which requires either 50 speech or 50 barter .

Bruce Isaac: In a motel room in Novac.

The Lonesome Drifter: North of El Dorado Gas & Service next to a Sunset Sarsparilla billboard.

once you have recruited every one go back to tommy and talk to him and the achievement should pop up

As Stated before just talk to Tommy Torini in The Tops.
You must recruit all of them not just one, also when you do this make sure you go back and talk to Tommy after each one because if you recruit all of them and then return to talk to Tommy it will glitch and you will get all of the rewards, but the quest will not complete no matter how many times you talk to him.
I did all of them then talked to Tommy 3 times in a row including doing it after clearing cache and off line. Each time it glitched and the quest did not complete.
So as a side note! When you do this Side Quest save after each one just in-case you need to fall back for some reason and do one at a time.
This glitch may not happen in every case, but it is a heads up if anything.

Note: I have obtained this achievement on another account. It was compromised so I had to switch to another.