the side quest is called "For Auld Lang Syne" looks like the first thing you need to do is have Arcade Gannon join your party, then you need to ask Dr. Henry, Daisy Whitman, CannibalJohnson, Orion Moreno and Judah Kreger to come to this Remnants reunion. There's a five word phrase you need in order to enter the remnant bunker, the words are "old, dear, navarro, remember and friends." You still need to recruit all the people to join you though, most can be convinced just by talking to them, Dr. Henry however you'll need to complete another side quest to recruit "Guess Who I Saw Today" essentiall involves.
Agree to look for the source of the nightstalker mutation.
Explore the Charleston Cave system to the north.
Battle Nightstalkers and uncover the chewed stealth boy from a slain super mutant.
Bring it back to Henry, coax someone named Lily into being a test subject, then calm or deal with Keene.
Then keep the tests running, stop the tests or solve the Dr's problem.
Dr. Henry will now be willing to meet up for the reunion.
Cannibal Johnson can be found in the cave bearing his name, east of the main New Vegas region, locate the rocky outcrop and the small cavern.
Daisy Whitman can be found at the Novac Motel in the town of the same name.
Judah Kreger is on the streets of westside, close to Miguel's Pawnshop.
Orion Moreno lives in a small house on the other side of the strip perimeter wall, just southeast of the gun runners establishment.
Dr. Henry can be located over in the northwestern mountains, trek to Jacobstown, talk to Marcus the super mutant and then enter the lodge.
After the gang's all been gathered go to the bunker almost due east of Jacobstown in the northwestern mountains, the correct sequence of previously mentioned words to get into the bunker is "Dear Old Friends Remember Navarro," once inside you have a choice between convincing the group to help you fight against caesar's Legion or NCR for control of the damn, if you convince them to fight for NCR against the Legion Orion Moreno takes offense and won't be a part of it, you can either talk to him or fight him, either way you get remnants power armor and helmet and a door over ride card and the chance for power armor training, if you haven't gotten it already. If however you choose to fight for Caesar against NCR, then Cannibal Johnson won't be happy, either convince or kill him, either way you get the same stuff, minus the door over ride card, only you can get power armor training from Moreno.