The Whole Gang's Here

 is a trophy/achievement in Fallout: New Vegas which is awarded for successfully convincing all of the companions to join you.

Possible bugs aside, the eight companions you need for this achievement are: Arcade Gannon, Craig Boone, ED-E, Lily Bowen, Raul Tejada, Rex, Rose of Sharon Cassidy, and Veronica Santangelo.

HD Video Guide.

Bugs

 * This trophy is apparently bugged and instead of hiring all of the companions, all you need to do is hire/rehire any companion as many times as there are companions. Verified for Xbox.
 * The achievement may glitch, to where you will not unlock it after you find them all. The easiest way to attempt to fix this is to have them all go to the same place (Lucky 38 suite recommended) and just constantly asking them to follow you, then return to Lucky 38 again.
 * Instead of hiring all companions, it may unlock simply by meeting all them and not hiring any.
 * If you chose to kill Rose of Sharon Cassidy during they Birds of a Feather quest, you will still be able to unlock the achievement.
 * You do not need Rex in order to get this achievement.
 * Achievement Bug on 360. When recruiting Rex i used the Legion Dog's Brain and Boone threatened to leave. I convinced him to stay but he would bug out when i had him follow me around. He would come up to me and talk to me automatically every 5 secs. I recruited all of the companions and didnt get the achievement. I did it again but didnt use the Legion Dog Brain and got the achievement.