The best way I've found to slice for serving larger groups is to slice and hold in hot au jus.
What I do is prepare some beef broth or au jus. You can buy "au jus base" at GFS or other places. It comes in a paste. You add it to boiling hot water and dissolve. If desired, you can doctor up just straight beef broth too. I've done this as well. Just heat up some beef broth and add some powdered garlic, onion, butter, etc. until you get the taste you're wanting. I'll even add in the drippings from the foiled brisket (if I have them).
Rest and slice the brisket like normal, taking care to keep the slices together to prevent oxidation of the slices and drying out. Put the slices tightly together in a pan (I usually use half foil pans...full pands will get really heavy and risk buckling if moving around). Once I have the slices all packed in tight, I'll pour the hot au jus over the slices until almost or just covering.
This does two things for me (at least in my mind)....
It will keep the slices hot AND hydrated while holding.
When people pick out their slices, they will be dripping with moisture.
No, it's not exactly the same as freshly sliced, right on the board, like they'd get if you were slicing for service, but it's still juicy, hot, moist brisket. It's definitely better than dry, oxidized, half cool brisket.
I've done this ahead of time, used shrink wrap and then foil lids, put them in the Cambro, transported to an event, and THEN served and the brisket was still hot and juicy.
The big thing is to prep the au jus and have it really hot before you slice the brisket.
You don't want to slice the brisket and THEN realize you have to prep and heat the au jus while the slices are drying out.
Have it all ready and on the stove, then as soon as you pan the slices, soak em.
Hope that helps.