How to properly reheat my brisket without it drying out?

First- do not slice the brisket before you get there. Period.

Next-What's the kitchen/grill situation when you get there? 

I'm going to use my new Anova precision cooker  sous vide to reheat my frozen leftovers - I'm guessing it'll be very good 

Put them (whole, uncut) in some disposable aluminum baking pans, on a rack.  Add water to the level of the rack.  Cover with foil being careful to avoid touching the meat with the foil but cinch it down over the baking pan.  Throw in the oven at about 400F.  It will steam.  You want to heat the stuff up to somewhere between 120-140F.  If it was cold, it will probably take a couple hours.  Make sure you don't run out of water.

There are many ways to reheat - this is a technique frequently used for pastrami (frequently brisket) in delis.

Low heat like 200 for a lomg time like 2 hrs.  You will be fine....

You could cut your stress level by 80% by just doing pulled pork and chopped brisket.  Bag it up and keep it in 140 degree water and serve it in steam pans. That way people can serve themselves and you can enjoy the party and refill pans in 30 seconds as they get low. As stated above, pork is still very good this way and your guests will tell you it's the best they have ever had. Same for brisket. We did this for a wedding with 300 guests and served them all at the same time with 4 people. If you want to serve sliced brisket, i would slice it at home and vac seal it and refrigerate it. At the party, Keep it in warm water with the pork and Put it out a little at at time maybe with a little sauce in the steam pan so it does not dry out. You will be managing water levels in the steam pan but this should still give you some time to socialize, have a fun New Year's Eve, This will require you to have a sous vide immersion circulator and a cooler. If you are planning on cooking for groups this large, those things will be invaluable to you anyway. 

My advice is to keep it as simple as you can and enjoy the party. I've been there and done that many times and it's always an ass-kicker when everyone else is having fun.

if it were me and this was the menu,  my prep and set up would be:

At Home-

Cook everything, have it done the day before the party. Pull/chop the pork and brisket and bag them in 5 lb packs with a little sauce and refrigerate.I would never do sliced brisket for a party when people are just grazing. It loses integrity very fast when sliced. 

At the party- arrive 2-3 hours early to set up.Fill a large cooler with hot water. Confirm there is a large stockpot available for you to heat water on the stove (don't use all their hot water to fill a cooler)

put your immersion circulator in at 140 and drop in the bags. They will need a couple of hours to heat throughout at 140. 

As the meat is warming, Set up a long table with a tablecloth with 3 steam pans (this is for the meat only- if you are serving warm sides, plan accordingly). You can buy steam pans for cheap at costco, sams, restaurant depot.you can also rent them for like $10 a day fat any party rental place (tables too if you don't have one). 

about 30 minutes before guests arrive, fill with water and light the fuel for the steam pans. When they start to steam, put about 10lbs of meat in each one and cover. In the 3rd steam pan you will have simple white buns for people to make sammies if they want. 

You will be filling pans pretty regularly for the first hour or so and then it should slow down once people are full and getting their drink on. 

You can't serve 100+ people over many hours by just putting meat out at room temp. You are going to need the steam pans no matter what. This is a safety issue aand a quality issue as well.

good luck- didn't mean to be a downer before. I've just done this so many times I always like to give people an out. IT's hard ass work and you are slaving away for hours while everyone else is having fun. Then they all leave and you are cleaning up until 2am. I'm always excited to do it, then halfway through I'm like good god what have I done