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 and PS3.
 * 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.
 * You do not need Rex in order to get this achievement.