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.

You do not have to get all the companions in one game. Over the course of several games you can successfully recruit all companions and get the achievement. There is a youtube video on this,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. After getting all companions on separate playthroughs and annoyingly not getting the achievement I found this technique wouldn't work with some characters but does with Raul; on PS3 with most recent update (26.11.10); peoples posts on the net suggest this is very successful.
 * 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.
 * On the PC, all companions including Rex are required for the achievement. However, despite receiving the achievement, it may not show up in Steam. Achievement was given after dismissing and then rehiring Raul a few times (after hiring all other companions through different games).
 * This achievement was awarded without ever meeting Veronica or in fact ever finding 188 Trading Post.
 * This achievement was awarded without ever meeting Cass.
 * This achievement was awarded without ever meeting Craig Boone